[browser][coreCLR] async main, thread pool, timers

[pavelsavara]

work in progress:

• -sEXIT_RUNTIME=0 for coreRun too
• don't call coreclr_shutdown2_func in coreRun/browserHost
• new AsyncHelpers.Browser.cs with
  • ExecuteAsyncEntryPoint that would call SystemJS_RejectMainPromise or SystemJS_ResolveMainPromise
• change Assembly::GetEntryPoint to find original async Main()
  • the roslyn wrapper contains blocking TaskAwaiter<int>.GetResult() which is not possible on ST browser.
  • use MemberLoader::FindMethodByName by original name, but ignoring return type
• change ValidateMainMethod to allow for signatures with Task or Task<int>
  • detect new CorEntryPointType.EntryManagedMainAsync and CorEntryPointType.EntryManagedMainAsyncVoid
• new SystemJS_ExecuteTimerCallback and SystemJS_ExecuteBackgroundJobCallback
  • ideally this would be [UnmanagedCallersOnly]
  • into src\coreclr\vm\wasm\helpers.cpp and src\coreclr\vm\corelib.h for now

Fixes#120212
Fixes#120211
Fixes#121046

This work with the PR

publicstaticasyncTask<int>Main(){Console.WriteLine("Hello, World! "+DateTime.Now);awaitTask.Delay(1000);Console.WriteLine("After Task.Delay() "+DateTime.Now);return42;}

TODO

• implement and test also Task Main() of void signature.
• make sure that synchronous main still works
• test exception marshaling
• fix browserHost
• test Release version JS name mangling
• process commandline args

[pavelsavara]

is there more feedback ?

[gewarren]

Left some doc suggestions.

[jkotas]

LGTM (the parts that I understand). Somebody with understanding of the JavaScript layer should review and signoff on that part.

[pavelsavara]

/ba-g CI timeouts and #114924