[whatwg/streams] Make read requests abortable (#1103)

Noted by @reillyeon in https://github.com/WICG/serial/issues/112#issuecomment-763936183 .

For some cases it would be nice to dequeue (and possibly abort?) a given read request, without canceling the stream entirely. That is,

```js
const controller = new AbortController();

// Assume "rs" contains chunks A and B.
const reader = rs.getReader();

const promise1 = reader.read();
const promise2 = reader.read({ signal: controller.signal });
const promise3 = reader.read();

controller.abort();
```

The intent here is that `promise1` gets fulfilled with chunk A, and `promise3` gets fulfilled with chunk B.

Some questions:

- What does this do to `promise2`? Probably reject it with an `"AbortError"` `DOMException` (like in other `AbortSignal` scenarios).
- What happens if abort is signaled after the read request is already fulfilled? Probably nothing (like in other `AbortSignal` scenarios).
- Does this do more than just dequeue the read request? For example, could it be communicated all the way down to the underlying source, I guess via `pull()`? I don't think this helps much, but it's worth checking...

This is looking relatively straightforward to me to spec, and the use case @reillyon presents is a good one. Should we just go for it?

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