Re: [whatwg/streams] WritableStreamDefaultController.abortReason (#1165)

I think if this is properly integrated into all calling specifications, such that calling `.abort(x)` causes methods like `fetch()` to reject with `x` instead of with an `"AbortError"` `DOMException`, then it would be good.

It would also solve the issue discussed in https://github.com/whatwg/dom/issues/951 about `"TimeoutError"` `DOMException` being most appropriate. /cc @annevk. We would make `AbortSignal.timeout()` call the spec equivalent of `controller.abort(new DOMException("...", "TimeoutError"))`.

The spec changes would probably be something like:

- Easy changes to the base primitives:
    - Each `AbortSignal` gets an **abort reason** JavaScript value, initially undefined (or null?).
    - `AbortSignal` gets a `reason` getter that returns this's abort reason.
    - `AbortController`'s `abort()` gets an `optional any reason` property. If it is not passed, it creates a new `"AbortError"` `DOMException`. It stores the `reason` in the abort reason for the created signal.
- Changes to all the callers:
  - Fetch:
      - all calls to "Abort fetch" which are triggered by an `AbortSignal` need to use the signal's abort reason, instead of creating a new "AbortError"
      - maybe also the stream erroring step?
  - Streams:
      - ReadableStreamPipeTo needs to consult the signal's abort reason
      - Remove `abortReason` stuff
  - Need to also update per https://dontcallmedom.github.io/webidlpedia/names/AbortSignal.html:
    - Web NFC
    - Web Bluetooth
    - App History
    - EyeDropper
    - Idle Detection
    - Prioritized Task Scheduling
    - Web Locks
    - Credential Management
    - Geolocation Sensor

This is a good amount of work, especially updating all the dependent specs/implementations/tests. But I think if we want to do this, we need to go the extra mile and update those too; being in an inconsistent halfway state where `.abort(reason)` only works with Streams would be bad.

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