Dates are in fact JSON-serializable, but as strings.

Recent changes to `isMessageEqual` mean that the `JSON.stringify` calls
cannot fail, because the `isMessage` check ensures all the fields we
serialize are strings. Writing a test to check for this failure mode is
not possible. There is a refactoring hazard here, so I added a comment
to call this out.

When comparing a MessageEvent's data to an expected message, only
compare the known fields of the message and ignore any unknown fields.

Unknown fields don't cause problems for us in parsing the message, and
there is a risk that it might cause us to silently ignore messages in
some environments. We don't know yet if that actually happens.

Previously the boot script was not regenerated when its JS inputs
changed, as opposed to the JSON manifest.

This approach relies on `src/boot` not having dependencies on other
modules. We could improve this by using `watchJS`, but that currently
produces too much distracting log noise when running `make dev`.

It was previously served as text/html, which caused warnings in some
contexts.

Update the representative feature tests to check for Shadow DOM v1 and CSS grid.

This avoids loading the client in browsers that are definitely too old.

Fix a UI confusion by fully un-setting the user-focus filter if a
empty user value is provided to the `changeFocusModeUser` RPC method.

This prevents a confusing half-set user-focus filter from sticking
around when an instructor returns to viewing all users in grading mode
after focusing on one or more students.

Bumps [sass](https://github.com/sass/dart-sass) from 1.43.5 to 1.44.0.
- [Release notes](https://github.com/sass/dart-sass/releases)
- [Changelog](https://github.com/sass/dart-sass/blob/main/CHANGELOG.md)
- [Commits](https://github.com/sass/dart-sass/compare/1.43.5...1.44.0)

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Bumps [@hypothesis/frontend-build](https://github.com/hypothesis/frontend-build) from 1.1.0 to 1.2.0.
- [Release notes](https://github.com/hypothesis/frontend-build/releases)
- [Changelog](https://github.com/hypothesis/frontend-build/blob/main/CHANGELOG.md)
- [Commits](https://github.com/hypothesis/frontend-build/compare/v1.1.0...v1.2.0)

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These tests access the `error.stack` property, which in turn triggers the
`Error.prepareStackTrace` [1] handler installed by source-map-support [2]. This
is expensive and has caused some CI timeouts.

The workaround is to disable the `error.stack` processing for the duration of
these tests.

[1] https://v8.dev/docs/stack-trace-api#customizing-stack-traces
[2] https://github.com/evanw/node-source-map-support/blob/ac2c3e4c633c66931981ac94b44e6963addbe3f4/source-map-support.js#L584

Bumps [puppeteer](https://github.com/puppeteer/puppeteer) from 12.0.0 to 12.0.1.
- [Release notes](https://github.com/puppeteer/puppeteer/releases)
- [Changelog](https://github.com/puppeteer/puppeteer/blob/main/CHANGELOG.md)
- [Commits](https://github.com/puppeteer/puppeteer/compare/v12.0.0...v12.0.1)

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Bumps [preact](https://github.com/preactjs/preact) from 10.6.1 to 10.6.2.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.6.1...10.6.2)

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 - versionify was removed as part of the Browserify => Rollup transition
 - We now use URLSearchParams instead of the query-string/querystring
   packages for query string serialization and parsing

Bumps [@hypothesis/frontend-shared](https://github.com/hypothesis/frontend-shared) from 4.3.0 to 4.4.0.
- [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/v4.3.0...v4.4.0)

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Bumps [aws-sdk](https://github.com/aws/aws-sdk-js) from 2.1038.0 to 2.1043.0.
- [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/v2.1038.0...v2.1043.0)

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Bumps [core-js](https://github.com/zloirock/core-js) from 3.19.1 to 3.19.3.
- [Release notes](https://github.com/zloirock/core-js/releases)
- [Changelog](https://github.com/zloirock/core-js/blob/master/CHANGELOG.md)
- [Commits](https://github.com/zloirock/core-js/compare/v3.19.1...v3.19.3)

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Bumps [rollup](https://github.com/rollup/rollup) from 2.60.1 to 2.60.2.
- [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/v2.60.1...v2.60.2)

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Bumps [eslint](https://github.com/eslint/eslint) from 8.3.0 to 8.4.0.
- [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/v8.3.0...v8.4.0)

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Bumps [prettier](https://github.com/prettier/prettier) from 2.5.0 to 2.5.1.
- [Release notes](https://github.com/prettier/prettier/releases)
- [Changelog](https://github.com/prettier/prettier/blob/main/CHANGELOG.md)
- [Commits](https://github.com/prettier/prettier/compare/2.5.0...2.5.1)

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Follow up of #3983, see:

* https://github.com/hypothesis/client/pull/3983#discussion_r760445342

* https://github.com/hypothesis/client/pull/3983#discussion_r760445894

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

Following the removal of `AnnotationSync` from `Guest` we correct the
tests.

AnnotationSync utility didn't have a clear purpose, except for assigning a
temporarily `$tag` to newly created annotations. For that, it used an
internal cache of annotation objects from the Guest, that were mutated
in-place. `AnnotationSync` used a very indirect method of communication
to the `Guest`, using an event emitter, even though their mutual
relationship was quite straightforward.

In this PR, we have:

* Simplified the messages that are sent between the host and the
  sidebar. We have assumed that a `$tag` value is always present if it
  comes from the `sidebar`, and a temporarily `$tag` is assigned in the
  `Guest` class for newly created annotations. Therefore, the type of
  the message sent between these two frames can be simplified to always
  be of the shape of `AnnotationData`, a safe version of `Annotation`.

* Added a few missing types.

* Changed the RPC method `deleteAnnotation` to send astring tag, similar
  to other RPC methods, like `focusAnnotations` or `scrollToAnnoation`.
  As a consequence of that we changed the `Guest#detach` method
  signature.

Currently only Chrome supports performing a structured clone of Errors.
Implement a fallback so that we can get information about annotator-side
errors in other browsers.

Also improve handling if a non-Error value is passed to `sendError`.

The `crossorigin="anonymous"` attribute was added to `<link
rel="preload">` elements created by the boot script because it was
needed when preloading API responses, so that the response can be read.

We don't need to use cross-origin requests when preloading stylesheets
however, and we were seeing occassional errors in Chrome due to this
(see https://github.com/hypothesis/client/issues/3987).

Previously `sendError` was tested indirectly via captureErrors. However
since this is exported as an API from the module, it makes sense to test
it directly.

Capture errors that happen while handling port requests in `PortProvider#listen`
and forward them to the sidebar, so we can gain visibility into them.

Add infrastructure that will allow us to gain visibility into errors
happening in the host frame, in specifically wrapped blocks of code, by
capturing the errors and forwarding them to the sidebar.

The initial implementation requires that the browser is able to clone
errors in `window.postMessage` calls, and currently only Chrome supports
that. In other browsers forwarding errors will fail with a warning.

 - Add `shared/frame-error-capture.js` module with functions for
   capturing errors, registering a target frame to receive them and
   forwarding errors to the frame.

 - Register sidebar application as handler for errors in host frame in
   `Sidebar` constructor and de-register it in the destructor.

 - Call `handleErrorsInFrames` in `sidebar/util/sentry.js` to handle
   errors from host frame by sending them to Sentry.

There is so much modification of the `menu-arrow` mixin that it's
not worthwhile abstracting it out (and we're moving away from mixins).

Update menu-arrow styling rules to accommodate.

Use `LabeledButton` instead of a `<button>` element in AdderToolbar
buttons, and extract pattern to `frontend-shared` SASS module.

Move the `frontend-shared` module up a level so it can be used by
annotator styles.

In `Ref<T>` the `current` property is now a readonly field. The correct
type (`MutableRef<T>`) is inferred automatically, so we don't need to
specify it.

Bumps [preact](https://github.com/preactjs/preact) from 10.5.15 to 10.6.1.
- [Release notes](https://github.com/preactjs/preact/releases)
- [Commits](https://github.com/preactjs/preact/compare/10.5.15...10.6.1)

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Call `listen` immediately after creating the PortProvider, before
creating the sidebar.

This will allow us to rule out the possibility that the sidebar sometimes fails
to connect to the PortProvider because an error occurred in the annotator code
after creating the sidebar iframe but before calling `PortProvider.listen`,
resulting in the PortProvider listener not being created.