cloudflare
Implement support for JS-backed WritableStreams in JSRPC
First pass at implementing support for JS-backed WritableStreams with JSRPC.
Provides an alternative adapter that will acquire the isolate lock on each write/close call, passing the operation off to the JS interface. This is obviously not as efficient as the original adapter used for WritableStreamSink but not yet sure if there's a better approach.
Reminder that GitHub loses information about force-pushes when you do two in a row which makes it hard to incrementally review. I think I've figured out that this diff represents the changes since I last reviewed, but in the future can you please post a list of fixup commits? The fixup.sh script in the internal repo generates such a list automatically if you use it.
