[whatwg/webidl] Update ongoing promise in async iterator return() method (PR #1387)

@yuki3 found [an interesting edge case](https://chromium-review.googlesource.com/c/chromium/src/+/5263418/6#message-d786cceaee26217e9a67d9201f5e10d1c0c4a34e) related to async iterators.

In Firefox and Node.js (where async iteration on `ReadableStream` is already supported), when you run this snippet:
```js
const readable = new ReadableStream({
  start(c) {
    c.enqueue("a");
    c.enqueue("b");
  },
  cancel() {
    console.warn("cancelled");
  }
});

const it = readable.values();
const p1 = it.next().then(x => console.log("next 1 resolved with", x));
const p2 = it.return().then(x => console.log("return resolved with", x));
const p3 = it.next().then(x => console.log("next 2 resolved with", x));
```
you get:
```
next 1 resolved with { value: 'a', done: false }
cancelled
next 2 resolved with { done: true, value: undefined }
return resolved with { done: true, value: undefined }
```

This is quite surprising: the second `next()` call resolves *before* the `return()` call, even though it was called *after* `return()`. Intuitively, we would expect all async iterator calls to get queued. Indeed, if you do the same thing with an async generator:
```js
async function* test() {
  try {
    yield "a";
    yield "b";
  } finally {
    console.warn("cancelled");
  }
}

const it = test();
const p1 = it.next().then(x => console.log("next 1 resolved with", x));
const p2 = it.return().then(x => console.log("return resolved with", x));
const p3 = it.next().then(x => console.log("next 2 resolved with", x));
```
you get the results in the same order as they were called:
```
next 1 resolved with { value: 'a', done: false }
cancelled
return resolved with { value: undefined, done: true }
next 2 resolved with { value: undefined, done: true }
```

Yuki and I believe this to be a bug in the Web IDL specification. More precisely, the `return()` method should update the ongoing promise, such that future calls to `next()` and `return()` are properly chained. This aligns more closely with the behavior of async generators.

---

- [ ] At least two implementers are interested (and none opposed):
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   * WebKit: …
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   * Node.js: …
   * webidl2.js: …
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(See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/webidl/pull/1387

-- Commit Summary --

  * Update ongoing promise in async iterator return() method

-- File Changes --

    M index.bs (12)

-- Patch Links --

https://github.com/whatwg/webidl/pull/1387.patch
https://github.com/whatwg/webidl/pull/1387.diff

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