Re: [whatwg/streams] Handeling backpressure on the Readable side of a transform stream (Issue #1323)

@ricea I really appreciate you taking the time to answer my question.
Sorry again for posting the question here, but this is so specific I really doubted anyone on StackOverflow would answer it.

Your example really helped me to understand some things better after which I re-read the spec one more time.

Your example actually gave me some inspiration, and I think I have thought up a simpler way to do it, what do you think about:
```javascript
function MyTransform() {
  let identityTransform = new TransformStream();
  const readable = identityTransform.readable;
  const identityWritter = identityTransform.writable.getWriter();
  const writable = new WritableStream({
    async write(chunk) {
      await identityWritter.ready;

      const foundTextIndex = chunk.indexOf('text to replace');

      if (foundTextIndex === -1) {
        identityWritter.write(chunk);
        return;
      }

      await identityWritter.ready;

      // Enqueue text before the part that needs to be replaced
      identityWritter.write(chunk.slice(0, foundTextIndex));

      // Enqueue the text from the second source
      // this is the part I'm not sure for which how to handle if we are adding
      // chunks to the readable sides internal queue even if it's full
      for await (const chunk of streamForReplacement) {
        await identityWritter.ready;
        identityWritter.write(chunk);
      }

      await identityWritter.ready;

      // Enqueue text after replacement
      identityWritter.write(chunk.slice(foundTextIndex + 'text to replace'.length));
    },

    close() {
      identityWritter.close();
    },

    abort(err) {
      identityWritter.error(err);
    },
  });
  return { readable, writable };
}
```

This way I'm using the identity transforms writter in order to respect the backpressure, since we can await `identityWritter.ready;` to know if we can write again

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