Re: [whatwg/streams] Cancelling and exception handling async iteration of ReadableStreams (Issue #1255)

@domenic it's basically unsafe to use async iteration on anything that acquires resources, unless you do something like wrap it with `abortableAsyncIterator`:

```ts
const abortable = <T,>(promise: Promise<T>): Promise<T> =>
  Promise.race([promise, this.abortedPromise as Promise<T>])

const abortableAsyncIterable = <T,>(
  iter: AsyncIterable<T>
): AsyncIterable<T> => ({
  [Symbol.asyncIterator]: () => {
    const inner = iter[Symbol.asyncIterator]()
    return {
      next: () => abortable(inner.next()),
      return: inner.return,
      throw: inner.throw,
    }
  },
})
```

I think this should be a builtin function or even special syntax like
```ts
for await (const elem of asyncIterable until signal) {
  ...
}
```

Because it's easy to forget that we'll leak resources without doing this, and I'm sure a lot of JS developers don't even realize it.

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