[whatwg/dom] Add a way to identify Abort errors originating from controllers (Issue #1033)

Hey,

I recently realized that it's now impossible (since `.reason` and `.abort(reason)`) to identify `AbortError`s, it would be really useful if there was some way to identify them as originating from cancellation (timeout is cancellation in that it's not an exceptional case that should warrant monitoring probably).

It would be useful either to have a property on `DOMException`s that signal cancellation/timeout users can check _or_ a branding check `AbortSignal.isAbortError` or similar.

This would enhance the ergonomics of using `AbortSignal`s while allowing the flexibility of passing `reason`.

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https://github.com/whatwg/dom/issues/1033