[whatwg/streams] Synchronous queue draining for WritableStream (#465)

A WritableStream invokes its underlying sink's `write()` method for each chunk in its queue. This completes only asynchronously on invocation of the fulfillment callback set to the promise returned by the `write()` call. This means that one microtask is required to process each chunk.

We could change this to allow for synchronous draining by introducing an interface similar to the `byobRequest` interface we've adopted for responding to BYOB reading in the underlying source of ReadableStream.

It'll be like:
- when a new chunk is enqueued, `write()` (or could be renamed to `push()`) is invoked to notify the sink of that there're some chunks available for processing (redundant invocation is still prevented by the `_writing` flag).
- WritableStreamDefaultController has a getter named `writeRequest()` which has `get chunk()` and `ack()` method.
- Calling `ack()` tells the WritableStream to fulfill the promise returned on `write()` call on the WritableStream.
- Once `ack()` is called, `writeRequest()` is updated to represent the next chunk in the queue. If there's no chunk, it returns **undefined**.


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