[browser][mt] Do not allow blocking on the thread with JS interop: UI, JSWebWorker

[lambdageek]

Blocking synchronization primitives should continue to throw on the

• UI, JSWebWorker threads
• PNSE as they do in single-threaded Wasm

There should be opt-out MsBuild flag, for projects who need to risk it.

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Issue Details

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Author:	lambdageek
Assignees:	-
Labels:	area-System.Threading
Milestone:	-

Tagging subscribers to 'arch-wasm': @lewing
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Issue Details

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Blocking synchronization primitives should continue to throw on the main thread as they do in single-threaded Wasm

Author:	lambdageek
Assignees:	-
Labels:	arch-wasm, area-Threading-mono
Milestone:	-