[whatwg/streams] GenericTransformStream mixin vs. subclassing (#1074)

https://github.com/whatwg/streams/pull/1073 works to formalize and formally recommend the pattern used by https://encoding.spec.whatwg.org/ and https://wicg.github.io/compression/, of wrapping up an internal `TransformStream` and re-exposing it through `readable` and `writable` properties.

However, I've lost track of why we do this, instead of subclassing `TransformStream`.

NativeFS shows [how to subclass `WritableStream`](https://wicg.github.io/native-file-system/#api-filesystemwritablefilestream), and #1073 makes it much easier by allowing you to reduce much of the boilerplate there to `[=WritableStream/Set up=] [=this=] with |writeAlgorithm| and |closeAlgorithm|`. It seems we could similarly make subclassing `TransformStream` easy.

This isn't very consequential. In general I expect the most noticable effect to be on `instanceof`. I don't expect us to add any significant public APIs to `TransformStream` that we'd want subclasses to inherit.

However, I am a bit concerned about recommending a totally different pattern for `TransformStream` than we do for `WritableStream` and `ReadableStream`. So I thought it'd be worth raising, and seeing if perhaps we want to change course.

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