This class is used by elements created by placeholder.js in the host
page, not a shadow root.

With the exception of highlight styles, the styles in annotator.css are
now only used by UI elements contained within shadow roots. As a result,
we can stop loading annotator.css as a stylesheet into the host page,
preventing these styles from affecting the host page.

 - Stop loading annotator.css into the host page as a stylesheet.
   Instead add it as a preload link instead, ready for use by shadow
   roots.

 - Move highlights styles into a separate bundle and load that in the
   host page.

Fixes https://github.com/hypothesis/client/issues/2979

All of our target browsers now support shadow DOM [1]. Make it mandatory so it
is clear that all annotator UI elements can rely on Shadow DOM and thus don't
need to worry about their internal styles affecting or being affected by the
host page.

[1] https://caniuse.com/?search=shadow%20dom

Co-authored-by: Robert Knight <robertknight@gmail.com>

There are two methods to get the host port from `PortProvider`:

- `#hostPortFrom` which returns the host port exclusively from the
  `sidebar-host` channel.

- `#on` allows to register a listener that is fired upon requests
  of host frames for channels other than the `sidebar-host`.

This is an attempt to simplify the API of `PortProvider` and merge both
methods into one.

Advantages:

- simplifies the `Sidebar` constructor signature (no need to pass the
  host port)

- avoids the pitfall of creating host-sidebar RPC channel before
  registering the RPC methods

- prepares for future guest-host channel see https://github.com/hypothesis/client/blob/ad93317debdac0d2c3abec83eb9d690934632ef3/src/annotator/index.js#L64-L66 (where the `#on` method is use to create the guest-host channel).

Co-authored-by: Robert Knight <robertknight@gmail.com>

1. I converted `_setupSyncToGuest` and `_setupSyncFromGuests` into
   conventional class methods.

2. Added a new method, `_setupHostEvents` for sidebar-host
   communication.

3. Invoke these three methods above, from the constructor.

The `PortProvider._channels` map was used only to keep track of which
channel requests have been handled for a particular frame. The
`WeakMap<Window, MessageChannel>` was in fact used as a set rather than
a map. Only the presence of entries affected behavior, not the stored
MessageChannel values.

Change the data structure and comments to make its purpose clearer.

This comment provides general information about how MessagePorts work,
but isn't directly related to any of the surrounding code.

Of the two comments here, the second one is the most useful information
to consumers of the PortProvider and PortFinder interfaces, but it is
well covered by eg. MDN's documentation on MessagePort.

Rework the code to make the code path for the sidebar-host channel more
similar to other channels. As a result the `_sendPorts` helper was only
called in one place and can be inlined where it is called to make the
overall logic easier to follow.

Update the `MarkdownEditor` component to use `Icon`, `IconButton`,
`LabeledButton` and `Link` where relevant. Reduce complexity of
local component CSS and use utility styles where possible. Extract
button and link style overrides to `frontend_shared` SASS module.

Part of https://github.com/hypothesis/client/issues/3876
Part of https://github.com/hypothesis/frontend-shared/issues/232

Bumps [eslint](https://github.com/eslint/eslint) from 7.32.0 to 8.3.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/main/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/compare/v7.32.0...v8.3.0)

---
updated-dependencies:
- dependency-name: eslint
  dependency-type: direct:development
  update-type: version-update:semver-major
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [rollup](https://github.com/rollup/rollup) from 2.60.0 to 2.60.1.
- [Release notes](https://github.com/rollup/rollup/releases)
- [Changelog](https://github.com/rollup/rollup/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rollup/rollup/compare/v2.60.0...v2.60.1)

---
updated-dependencies:
- dependency-name: rollup
  dependency-type: direct:development
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@babel/preset-env](https://github.com/babel/babel/tree/HEAD/packages/babel-preset-env) from 7.16.0 to 7.16.4.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/main/CHANGELOG.md)
- [Commits](https://github.com/babel/babel/commits/v7.16.4/packages/babel-preset-env)

---
updated-dependencies:
- dependency-name: "@babel/preset-env"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.1028.0 to 2.1034.0.
- [Release notes](https://github.com/aws/aws-sdk-js/releases)
- [Changelog](https://github.com/aws/aws-sdk-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/aws/aws-sdk-js/compare/v2.1028.0...v2.1034.0)

---
updated-dependencies:
- dependency-name: aws-sdk
  dependency-type: direct:development
  update-type: version-update:semver-minor
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.27.0 to 7.27.1.
- [Release notes](https://github.com/yannickcr/eslint-plugin-react/releases)
- [Changelog](https://github.com/yannickcr/eslint-plugin-react/blob/master/CHANGELOG.md)
- [Commits](https://github.com/yannickcr/eslint-plugin-react/compare/v7.27.0...v7.27.1)

---
updated-dependencies:
- dependency-name: eslint-plugin-react
  dependency-type: direct:development
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [reselect](https://github.com/reduxjs/reselect) from 4.1.2 to 4.1.4.
- [Release notes](https://github.com/reduxjs/reselect/releases)
- [Changelog](https://github.com/reduxjs/reselect/blob/master/CHANGELOG.md)
- [Commits](https://github.com/reduxjs/reselect/compare/v4.1.2...v4.1.4)

---
updated-dependencies:
- dependency-name: reselect
  dependency-type: direct:development
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [typescript](https://github.com/Microsoft/TypeScript) from 4.4.4 to 4.5.2.
- [Release notes](https://github.com/Microsoft/TypeScript/releases)
- [Commits](https://github.com/Microsoft/TypeScript/compare/v4.4.4...v4.5.2)

---
updated-dependencies:
- dependency-name: typescript
  dependency-type: direct:development
  update-type: version-update:semver-minor
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@sentry/browser](https://github.com/getsentry/sentry-javascript) from 6.14.3 to 6.15.0.
- [Release notes](https://github.com/getsentry/sentry-javascript/releases)
- [Changelog](https://github.com/getsentry/sentry-javascript/blob/master/CHANGELOG.md)
- [Commits](https://github.com/getsentry/sentry-javascript/compare/6.14.3...6.15.0)

---
updated-dependencies:
- dependency-name: "@sentry/browser"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [karma](https://github.com/karma-runner/karma) from 6.3.8 to 6.3.9.
- [Release notes](https://github.com/karma-runner/karma/releases)
- [Changelog](https://github.com/karma-runner/karma/blob/master/CHANGELOG.md)
- [Commits](https://github.com/karma-runner/karma/compare/v6.3.8...v6.3.9)

---
updated-dependencies:
- dependency-name: karma
  dependency-type: direct:development
  update-type: version-update:semver-patch
...
Signed-off-by: dependabot[bot] <support@github.com>

Bumps [@hypothesis/frontend-shared](https://github.com/hypothesis/frontend-shared) from 4.1.0 to 4.3.0.
- [Release notes](https://github.com/hypothesis/frontend-shared/releases)
- [Changelog](https://github.com/hypothesis/frontend-shared/blob/main/CHANGELOG.md)
- [Commits](https://github.com/hypothesis/frontend-shared/compare/v4.1.0...v4.3.0)

---
updated-dependencies:
- dependency-name: "@hypothesis/frontend-shared"
  dependency-type: direct:development
  update-type: version-update:semver-minor
...
Signed-off-by: dependabot[bot] <support@github.com>

The current version of `isSourceWindow` raised an error for (1) older
browser that doesn't support ServiceWorker and (2) web pages served from
non-secured http connections.

I tested this alternative fix but it didn't work for me in the latest
version of Chrome:
https://stackoverflow.com/questions/6229301/a-clean-way-of-checking-whether-an-object-is-an-instance-of-window-constructor/6229603#6229603

In future PR, `Guest` will contain two bridges (like `FrameSync`).
Naming them by the destination frame will help identify the purpose of
it.

It seems that the convention for this `Bridge`s is to contain the word
`RPC` to indicate that it is a remote (inter-frame) procedure.

This adoption simplifies the discovery of frames.

I did a little bit of clean-up in the tests: rearranged a few variables
alphabetically, and delete unneeded lines and variables.

This includes a fix for incorrect results under certain conditions when
the pattern length is an exact multiple of 32. See
https://github.com/robertknight/approx-string-match-js/pull/13.

This reverts commit 5d69a81c.

This reverts commit 2957944d.

This reverts commit 520b5c50.

In future PR, `Guest` will contain two bridges (like `FrameSync`).
Naming them by the destination frame will help identify the purpose of
it.

It seems that the convention for this `Bridge`s is to contain the word
`RPC` to indicate that it is a remote (inter-frame) procedure.

This adoption simplifies the discovery of frames.

I did a little bit of clean-up in the tests: rearranged a few variables
alphabetically, and delete unneeded lines and variables.

The tests crashed if either of these commands was executed:

- `yarn test --grep anchoring`, or
- `yarn test --grep 'anchoring/test/pdf|test/integration/anchoring'`

`src/anotator/anchoring/test/pdf-test.js` cleanup the Viewer by setting
the `window.PDFViewerApplication' to `null`. After that, if another test
created a `Guest` instance (like
`src/annotator/test/integration/anchoring-test.js`) will think that the
document is a PDF and will crash. This is the code that checks if the
document is a PDF:

https://github.com/hypothesis/client/blob/492ab6d56a5685356ca6b3a48644e047bb28dca0/src/annotator/integrations/pdf.js#L53-L56

The solution is either deleting `window.PDFViewerApplication' or setting
it to `undefined`.

We use `Window` objects as keys on a map. Windows can be removed.
`WeakMap` has the characteristic that keys that are claimed by the
garbage collector are removed from the map. In this way, we are memory
friendly and don't retain any defunct object.

Create a test page for PDF-based VS books, using content adapted from
https://bookshelf.vitalsource.com/reader/books/9781938168239/, an
OpenStax textboot that is available for free [1] in VitalSource.

 - Rename vitalsource.mustache => vitalsource-epub.mustache to make the
   distinction with the PDF example clearer.

 - Create vitalsource-pdf.mustache which serves as the "container" page
   for a PDF-based VS book. This is a copy of vitalsource-epub.mustache
   with the Prev/Next buttons removed and slight terminology changes
   ("chapter" => "page") to reflect how PDFs are handled in VS

 - Revise the descriptive text at the top of the VS PDF and EPUB
   examples to explain the purpose more clearly and relate them to how
   books are presented in the Bookshelf store, where the terms
   "reflowable" and "fixed" are used instead of "EPUB" and "PDF".

 - Add a very lightly edited copy of a real content page from a VS book
   in vitalsource-pdf-page.mustache. This is the HTML content from the
   iframe displaying a page in a PDF book in VS. The only changes I have
   made are small block of CSS customizations at the top (see comment)
   and a change to the URL of the rendered PDF page.

 - Update the links to the VitalSource test pages on the dev server.

[1] Even though the book is free, you have to add it to your VitalSource
    book library before you can access it. To do that, go to
    https://bookshelf.vitalsource.com and search for the book ID (9781938168239
    in this case), then click "Open Book".

Following these two comments:
* https://github.com/hypothesis/client/pull/3929/files#r750399422
* https://github.com/hypothesis/client/pull/3929/files#r750414036

I am proposing in this PR to remove the anti-collision field `authority`
from the message that is send to discover frames. I believe the message
contains enough information to avoid to be confused by other unrelated
`postMessage` in the host frame.

Co-authored-by: Robert Knight <robertknight@gmail.com>