[whatwg/webidl] Add an extended attribute to allow propagating TC39's AsyncContext (PR #1568)

This PR updates WebIDL to add the `[PropagatesAsyncContext]` extended attribute. This is a part of the TC39 AsyncContext proposal, which allows storing state associated with an async flow of execution in JavaScript, and preserving it across different kinds of async continuations in both JS and the web platform.

This `[PropagatesAsyncContext]` extended attribute can only be applied to callback function types in operation arguments. When set, the callback context will also store an Async Context Mapping, which the callback will run in.

This allows a callback-taking operation to act like an async continuation, and propagate the state associated with the async flow of execution at the time that this operation is called to the callback.

This patch relies on HTML's PR whatwg/html#12152 to define the "run with Async Context Mapping" operation. This operation must be defined in a web specs rather than in the TC39 proposal because it deals with throwing in spec algorithms, which is not a concept in the TC39 specs.

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You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/webidl/pull/1568

-- Commit Summary --

  * Add an extended attribute to allow propagating TC39's AsyncContext

-- File Changes --

    M index.bs (160)

-- Patch Links --

https://github.com/whatwg/webidl/pull/1568.patch
https://github.com/whatwg/webidl/pull/1568.diff

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