Re: [whatwg/dom] Define a 'CancellationController' and 'CancellationSignal' interface. (#434)

Per conversation with @annevk on IRC: I'd like to see a cross-realm way to unforgeably determine if the rejection reason is a cancellation or not - comparing to a string property, or using `Object.prototype.toString.call` output, is not sufficient, and neither is `instanceof`. In other words, for all the same reasons `Array.isArray` exists - which does a brand check that can't be faked or tampered with - it'd be ideal to have something like `CancelException.is()` or similar.

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