Access js Worker object after wasm_worker creation

## Background
We want to transfer a readable stream object (`postMessage(stream, {transfer: [stream]})`) to a wasm worker to be able to read the stream from the worker thread.

We intend to start a `message` listener through EM_JS like the following example, with the addition of checking that it is the right message and not an internal Emscripten one:
```js
addEventListener("message", async (event) => {
const reader = event.data.readableStream.getReader();
while (true) {
const frame = await reader.read();
if (frame.done) {
break;
}
wasmModule.insertFrame(frame.data);
}
});
```

But to post the message that we can listen to, you would need to access the `Worker` object in JS.

## Possible solutions

1. Be able to return `emscripten_wasm_worker_t` or similar in C++ and get get a `Worker` returned in JS.
2. Have a public lookup method on the module where the `id` of the Worker (`emscripten_wasm_worker_t`) can be used to fetch the corresponding Worker object.

Alternative solutions may exist, and any method of accessing the Worker object is acceptable.