[browser][MT] use regular POSIX portable threadpool

[pavelsavara]

Because we dispatch all interaction with JS to the thread which owns the JS objects,
we don't need to yield to browser event loop in the managed thread pool any more.

• drop WebWorkerEventLoop
• drop LowLevelLifoAsyncWaitSemaphore
• drop mono_thread_platform_external_eventloop_keepalive_check
• drop LowLevelLifoSemaphoreBase and _LifoSemaphoreBase

This is fine piece of machinery which I'm removing, but we don't need it for browser and it would be wasting time to maintain it.
I wonder if we will need to resurrect some of this for WASI, once preview 3 brings futures/promises. But I hope not and I hope that there would not be as strong thread affinity either.

Fixes #85052

[pavelsavara]

I filed #99888 for timeouts. I think it's not related to this PR, but I will explore further.

[lambdageek]

I wonder if it's worth keeping the async semaphore around. But I suppose we can always revert that part, if there is a need in theb future.

Threadpool changes lgtm.

Part of the work here originally was slicing up the event loop into multiple functions and files. @kouvel , would you prefer everything back into a single giant function, or no?

[kouvel]

Part of the work here originally was slicing up the event loop into multiple functions and files. @kouvel , would you prefer everything back into a single giant function, or no?

It would be nice to clean up some things if they're not necessary anymore (from a4e2609, dcb34de, 042f593). Eg:

• If PortableThreadPool.WorkerThread.CreateWorker.cs is not expected to be needed anymore, the code could be folded into PortableThreadPool.WorkerThread.cs and maybe also undoing some of the refactoring if it's not expected to be needed
• Similarly, if LowLevelLifoSemaphoreBase.cs is not expected to be needed anymore, the code could be folded into LowLevelLifoSemaphore.cs

Anyway, the thread pool changes LGTM aside from some cleanup that could be done.

[pavelsavara]

some cleanup that could be done.

Moved code back, no functional change. (apart from the first commit here which changes Browser TP)

[kouvel]

Moved code back, no functional change. (apart from the first commit here which changes Browser TP)

Thanks! Just left one minor suggestion but otherwise the moves LGTM.