Re: [whatwg/streams] Allow stream reader cancel() method to return bytes from queue instead of discarding them. (#1147)

@domenic, can you reopen this issue? If there's action to take here then it should be done at the Streams API level.

While it might be nice syntactically to have `cancel()` return any bytes left in the queue I think this can be polyfilled with,

```js
const finalReadPromise = reader.read();
await reader.cancel();
const finalRead = await finalReadPromise;
```

If there was any data left in the queue then it will be in `finalRead.value`. Otherwise, if the read was blocked waiting for more data then `finalReadvalue` will be `undefined`. The key insight here being that you can do a "non-blocking" read to see if there are queued bytes by not awaiting on the `Promise` it returns. Once you call `cancel()` then that `Promise` is guaranteed to have resolved one way or another.

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