Update async `Main` code generation

[333fred]

dotnet/runtime#121046 proposes a new api for async Main methods to allow WASM to understand the entry point correctly. If approved, we will want to react and update the codegen for our stub entrypoint methods in the case the user uses async Main:

static async void Main() { }

// Generated stub will now be:
static void <Main>(string[] args) => AsyncHelpers.ExecuteAsyncEntryPoint(Main(args));