stream: add diagnostics_channel event for completion

This adds a stream.web.done diagnostics channel to Web Streams. with the intention of having a straightforward way to detect when a ReadableStream has been read to completion, without triggering the reading of the stream.

This is useful for instrumentation tools trying to measure the length of a request/response cycle triggered by a fetch().

Why draft?

Right now I'm only testing directly calling read() and async iterators. pipeThrough(), pipeTo(), and cancellation all need to be tested (and perhaps implemented).

Also, the naming is certainly open for suggestions. I based the name of the channel on the only other prior art in Node.js core.

Finally, I had a weird thing happen with mksnapshot, so I'm wondering if anyone knows how to better work around that. See the inline comment below.

All that being said, I thought it was worth opening it now anyway, to make sure the approach is sound and agreeable.