Rewrite TransformStream using the public API

[MattiasBuelens]

This re-implements TransformStream to use the public API of ReadableStream and WritableStream, rather than using internal abstract ops. The idea is that, in the future, we could construct a polyfill TransformStream on top of a native ReadableStream and/or WritableStream.

This also adds partial support for WritableStreamDefaultController.signal.reason, which we use to detect when writable.abort(reason) is called. We do have to "cheat" a bit here, since AbortSignal.reason may not yet be supported everywhere and web-streams-polyfill does not attempt to polyfill this API by itself.

[jimmywarting]

I'm seeing all this _ prefix everywhere all over the pr and in all the files... why don't you just use # instead?

[MattiasBuelens]

I still ship a build for ES5, so I have to be very careful with what syntax features I can or cannot use. The down-level emit for private fields uses WeakMap, and I'm not 100% sure if that's safe to use everywhere... 😬

I could move everything into a semi-private Symbol("state") or something (like in Node), and replace Symbol with String on platforms that don't support it. Not sure yet. 😛

[jimmywarting]

I'm not sure if typescript can handle that down leveling for you, don't use typescript...
but i prefer the god old _ prefix over symbol cuz it's cleaner code.

[MattiasBuelens]

I could throw Babel into the mix and use the loose option instead. That changes the emit to generate a unique property name for the private field instead, which would work on ES5. See demo.

It would make the brand checks nicer though, since you can do #privateField in this instead and be guaranteed that this was constructed by your class. See docs. This ties in with #70 (comment).

Anyway, I'll leave that for another time. 😛