[browser][coreclr] implement async main and ST thread pool

[pavelsavara]

It would unlock following features that depend on yielding to browser event loop

• Threadpool
• GC Finalizer
• Timers
• HTTP & WS clients

implementation notes

• make BrowserHost_ExecuteAssembly to yield
• ICLRRuntimeHost4->ExecuteAssembly(domainId, managedAssemblyPathW, argc, argvW, (DWORD *)exitCode)
• src\coreclr\vm\corhost.cpp - CorHost2::ExecuteAssembly
• src\coreclr\vm\assembly.cpp - Assembly::ExecuteMainMethod
• RunMain -> ValidateMainMethod, RunMainInternal
• SystemJS_ScheduleBackgroundJob

        public static void MainWrapper(string[] args, Func<string[], Task<int>> original, Action<int> resolve, Action<string> reject)
        {
            var task = original(args);

            task.ContinueWith(t =>
            {
                if (t.IsFaulted)
                {
                    reject(t.Exception.Message);
                }
                else
                {
                    resolve(t.Result);
                }
            }, TaskScheduler.Default);
        }

runtime/src/libraries/System.Runtime.InteropServices.JavaScript/src/System/Runtime/InteropServices/JavaScript/JSHostImplementation.cs

Line 208 in 3149da8

public static unsafe Task<int>? CallEntrypoint(IntPtr assemblyNamePtr, string?[]? args, bool waitForDebugger)

runtime/src/mono/browser/runtime/corebindings.c

Line 115 in 3149da8

void SystemInteropJS_AssemblyGetEntryPoint (char *assembly_name, int auto_insert_breakpoint, MonoMethod **method_out)

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[pavelsavara]

src\coreclr\vm\corelib.h

[pavelsavara]

MethodDescCallSite::CallTargetWorker

[pavelsavara]

System.Threading.ThreadPool::MainThreadScheduleBackgroundJob