[whatwg/dom] Allow postMessage-ing AbortSignal (#948)

Currently communicating to a worker that work should be aborted is somewhat annoying. This is especially true for work that may be run in a synchronous loop as one can not simply send forward a `MessageChannel` that signals aborting, but rather you need to send a `SharedArrayBuffer` that can be synchronously read.

I'd like to propose the ability to `.postMessage` an `AbortSignal` so that we can communicate aborting to workers in a consistent way to aborting on the main thread. Ideally this would support the synchronous case as well (e.g. calling `controller.abort()` from the original thread would synchronously update `signal.aborted` in the other thread so that compute loops can simply do `if (signal.aborted) { /* terminate work */ }`).

If there's interest, I would be willing to create a PR for this.

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