* add information about why to use `window.load` instead of
  `document.DOMContentLoaded` to check when a document is ready

* use an invalid local URL (http://localhost:1) to speed DNS lookup in
  the test

* include some comments about the use of `waitForFrameObserver` in the
  tests

Instead of override the window.hypothesisConfig, I provide the
`externalContainerSelector` through a JSON script tag. This allows a
more incremental way to add configuration options.

When injecting the client into a guest frame, proxy any custom asset locations
specified via `.js-hypothesis-config` script tags in the host frame.
These are used by the browser extension and needed for the VitalSource
integration to work in that context.

We could probably avoid copying the `sidebarAppUrl` and `notebookAppUrl`
settings for guest-only frames. This will require changes to the boot
script and possibly the annotator entry point.

Implement the `injectClient` method of Guest that is used by
integrations (eg. VitalSource) to inject the client into a chosen frame.

In the process the `HypothesisInjector` class has been extracted out of
the `CrossFrame` class, since it is unrelated to the rest of the
functionality in that class and only lived their because of its
dependence on the `Bridge` instance, which will soon be removed (see
https://github.com/hypothesis/client/pull/3812). It is now constructed
and used directly by the `Guest` class instead.

Expose the existing logic for injecting the client into a target frame
in HypothesisInjector as a public `injectClient` method and modify it to
wait for the document to be loaded if necessary. This method will be
used by the guest class to inject the client into specific frames when
requested by the current integration.

In the case where the frame is discovered by `FrameObserver` this means
that `onDocumentReady(frame)` will be called twice. That's OK because
the second call will just complete immediately if the frame is already
ready.

Change how the sidebar is notified of guest frames being unloaded to
support guest frames where the client has been loaded via means other
than `HypothesisInjector` or where the guest is cross-origin.

Instead of listening for the guest frame's 'unload' event from the
parent frame in `HypothesisInjector`, the guest frame instead listens
for this event itself and sends a `hypothesisGuestUnloaded` message to
the host frame via `window.postMessage`, which in turn is handled in the
`Sidebar` class to relay it to the sidebar app via a `destroyFrame` RPC
call. This indirect route works around a bug in Safari (see code
comments).

As well as supporting future use cases, this also simplifies the
`HypothesisInjector` class as it no longer needs access to the `Bridge`.

This enables testing handling of the client's discovery of existing
frames (on page load) as well as handling of dynamic addition and removal after
the client has started.

Use separate channels for sending messages to guests vs the host in the
sidebar, even for the common case when there is only one guest and it is
the same frame as the host.

This change makes it clear for readers which part of the annotator code
is intended to handle a particular message from the sidebar. It is also
a step towards supporting host frames that are not guests. This will be
needed in ebook readers where the host frame provides the navigation UI
and contains the frame displaying the book content, but should not be
annotatble itself.

 - Remove the `bridge` service in the sidebar. The `frameSync` service
   now provides the entry point for other services/components to make RPC calls
   to the host or guest frames. Currently the only use case is
   sending notifications to the host via `FrameSyncService.notifyHost`.

 - Create separate `Bridge` instances in `FrameSyncService` for sidebar
   <-> guest and sidebar <-> host communication. The sidebar <-> guest
   bridge works the same as before. The sidebar <-> host bridge
   is established by having `FrameSyncService` create a MessageChannel
   when sending the `hypothesisSidebarReady` notification to the host.

   The sidebar / host frames then add respective ports of this channel
   to a Bridge instance.

 - Change the various existing RPC calls between frames to use either
   the guest <-> sidebar or host <-> sidebar communication channels as
   appropriate

Show the URLs of all frames connected to the sidebar in the Version info
tab of the Help panel. Previously only the URL of the main frame was
shown.

Initially this will mainly be useful for client developers to check that
the expected set of frames are connected to the sidebar in different
scenarios.

`Annotator` is not a great name for this interface, but at least make
the link between it and Guest clearer by improving its documentation and
marking Guest as explicitly implementing it.

This adds a new integration for the VitalSource Bookshelf reader. It is
comprised of two separate sub-integrations, one for the "container"
frame into which the client is initially loaded, and a separate one for
the "content" frame that displays the book content.

The container frame integration is responsible for injecting the client
into the content frame and re-injecting after a navigation. All the
annotation-related functionality is a no-op in this frame. The logic for
injecting the client depends on a stub method in the Guest class which
will be implemented separately.

The content frame integration mostly delegates to the HTML integration,
but it customizes the document metadata saved with annotations and also
prevents the VitalSource viewer's native selection controls from
interfering with Hypothesis's ones.

All frontend Hypothesis app are going to have the same base line as
this:

https://github.com/hypothesis/bouncer/pull/469/commits/6a01199bfc196a8932be271f61c07029dbf886c5

Add a `canAnnotate` method to the `Integration` interface to indicate
whether a range is part of the annotatable content of the document. This
allows integrations to signal whether the Annotate/Highlight toolbar
should be shown for a given selection.

Implement it for PDFs by returning `true` if the range is part of the
text layer of a single page. Implement it for HTML documents by always
returning true.

Fixes #3790

Bumps [@octokit/rest](https://github.com/octokit/rest.js) from 18.11.1 to 18.11.4.
- [Release notes](https://github.com/octokit/rest.js/releases)
- [Commits](https://github.com/octokit/rest.js/compare/v18.11.1...v18.11.4)

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

Bumps [eslint-plugin-react](https://github.com/yannickcr/eslint-plugin-react) from 7.26.0 to 7.26.1.
- [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/v7.26.0...v7.26.1)

---
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 [babel-plugin-mockable-imports](https://github.com/robertknight/babel-plugin-mockable-imports) from 1.7.1 to 1.8.0.
- [Release notes](https://github.com/robertknight/babel-plugin-mockable-imports/releases)
- [Changelog](https://github.com/robertknight/babel-plugin-mockable-imports/blob/master/CHANGELOG.md)
- [Commits](https://github.com/robertknight/babel-plugin-mockable-imports/compare/v1.7.1...v1.8.0)

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

`globalThis` is the preferred way of accessing the global object
regardless of environment. It also works in eg. Rollup whereas `global`
does not.

This is in preparation for switching to an ES module-oriented bundler.

Bumps [@actions/core](https://github.com/actions/toolkit/tree/HEAD/packages/core) from 1.5.0 to 1.6.0.
- [Release notes](https://github.com/actions/toolkit/releases)
- [Changelog](https://github.com/actions/toolkit/blob/main/packages/core/RELEASES.md)
- [Commits](https://github.com/actions/toolkit/commits/HEAD/packages/core)

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

Bumps [@hypothesis/frontend-shared](https://github.com/hypothesis/frontend-shared) from 3.11.0 to 3.12.0.
- [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/v3.11.0...v3.12.0)

---
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>

Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.995.0 to 2.999.0.
- [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/v2.995.0...v2.999.0)

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

Although the `delay` utility function is a small function that can
almost be in-lined, I decided to create a common `delay` function in
`wait.js` and replace all usage in tests by the new function.

This adds a mock version of the VitalSource Bookshelf reader to the dev
server. It includes only the essential aspects of the viewer's structure that
will trigger VitalSource-specific functionality.

The Guest implementation no longer has any logic which is specific to a
particular document type (PDF, HTML etc.). These details are now all
handled by the Integration implementation returned by
`createIntegration`.

This commit refactors the Guest tests to use a single Integration fake
instead of using different fakes to simulate HTML and PDF documents.

Move the logic for determining which document type/viewer integration to
create out of the annotator entry point and Guest and into a
`createIntegration` factory method in annotator/integrations/index.js

This reduces the number of places that need to change when we add new
integrations, and puts the logic for determining whether a particular
integration can be used in the current environment alongside the
corresponding implementation.

Because of the sometimes-arcane nature of grid layouts, it is sometimes
necessary to apply definite width values on grid areas (versus `auto`)
to prevent content from escaping.

This fixes a situation in which very long unbroken quoted excerpts could
blow out the column width in the Notebook.

See https://css-tricks.com/preventing-a-grid-blowout/

This was created to simulate a scenario in VitalSource's book reader,
but they changed their reader so that it no longer uses a tall frame.

This is a prerequisite for moving to a modern ES-module oriented module
bundler such as Rollup.

There is one remaining import ('preact/debug') which was not converted
because that is conditional, and top-level `import` statements can't be
conditional. That might need to use either `import(...)` or a different
approach.

This required adapting pattern library import paths due to changes in
https://github.com/hypothesis/frontend-shared/pull/200.