Sync-Api doesn't work with multiple workers

Suppose you have something like so:

Main Thread - Extension code
Worker 1 - Extension sub piece, which starts Worker 2
Worker 2 - Needs to use the sync api

Worker 2 cannot communicate with the Main Thread because its MessageChannel is only with Worker 1.

For CPython I have this exact situation when trying to run the debugger:

• Main thread - Extension code
• Worker 1 - Load of python.js
• Worker 2 - Reader thread for debugger
• Worker 3 - Writer thread for debugger

Worker 2 and 3 cannot send messages to the Main Thread.

• Note: Emscripten (by default proxies) all FS (or socket) API calls to its main thread (worker 1 in this case) but that causes deadlock issues because the Atomics.wait in the sync-api block the main worker thread, therefore no other fs API calls can be handled while waiting. Took me a while to figure this out as most of it was working until Worker 3 tried to acquire a lock.

Start by tracing the sync-api MessageChannel setup across the worker hierarchy described in the issue, then reproduce the CPython debugger scenario with Emscripten's filesystem proxying. Done means nested workers can communicate with the Main Thread without the Atomics.wait deadlock.