[browser][MT] make UI thread not managed/attached

[pavelsavara]

Currently the UI thread is attached Mono thread and can call into Managed code.
The downside is that it needs to enter GC barrier, which makes it participate in the stop-the-world.

Current blockers are

• reflection in init_managed_exports
• mono_wasm_gc_lock, mono_wasm_gc_unlock used by Blazor
• monoObjectAsBoolOrNullUnsafe used by Blazor
• monoStringToStringUnsafe used by Blazor
• mono_wasm_assembly_load, mono_wasm_assembly_find_class, mono_wasm_assembly_find_method by looking for entrypoint
• GC roots in hybrid globalization

Also emscripten will complain about blocking UI thread when ASSERTIONS are enabled.
Blocking on the main thread is very dangerous, see https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread

See https://github.com/emscripten-core/emscripten/blob/8c81cac1bbae378bc7dde0c21c99602cbaf452d0/src/library_pthread.js#L910-L922

If we are unable to do this, the alternative is #76963

[dotnet-policy-service]

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