Accept polyfilled abort signals

[MattiasBuelens]

Currently, the polyfill performs a brand check using the global AbortSignal.prototype.aborted (source). This makes it difficult to use the polyfill in a non-DOM environment where there is no global AbortSignal, and where AbortController and AbortSignal themselves need to be polyfilled.

This PR loosens the check, allowing any object with a boolean aborted property to be accepted as a valid AbortSignal. This is a small deviation from the specification, but makes the polyfill usable in more environments (e.g. Node).

In the future, if the polyfill is able to use the native streams implementation (#20), we must ensure that any polyfilled AbortSignal is first converted to a native AbortSignal before calling the native pipeTo/pipeThrough implementation.

[MattiasBuelens]

We still create a DOMException here, which also does not exist in non-DOM environments. I'm not sure if/how we want to polyfill that. 😕