Re: [whatwg/streams] Add an example of a transform stream that replaces template tags (#846)

Another proposed example for TransformStream. I like this, it's clearly useful and it demonstrates the use of flush() and how to deal with edge cases. But it has a few drawbacks:
- It's sufficiently tricky that it needs its own tests. We don't have a good place to put the tests. I've included them in this PR so you can inspect them.
- Some parts of the implementation may be tricky to understand.
- It's sufficiently useful that it needs a note warning you not to use it in production.

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