Feature request: MAIN_THREAD_EM_ASYNC_JS

`EM_ASYNC_JS` is really useful for writing async JS code with `await`, however it runs on the thread it was called on. It would be nice if there were a version that proxied the call to the main thread.

The alternative is to use `MAIN_THREAD_ASYNC_EM_ASM` (extra boilerplate with `Asyncify.handleAsync` and doesn't define a C function like `EM_ASYNC_JS` does):

```
MAIN_THREAD_ASYNC_EM_ASM({
return Asyncify.handleAsync(async () => {
// ....
});
});
```

and then setting `-s 'ASYNCIFY_IMPORTS=["emscripten_receive_on_main_thread_js"]'`

related https://github.com/emscripten-core/emscripten/issues/8991#issuecomment-846440750