Add an async method that ensures the worker has started

[webJose]

Motivation

It may be desirable to know whether the worker has already started. As a minimum this could help in testing.

Technical Specification

Consumers would add a method whose sole purpose is to confirm the worker is running, like ping:

epxort const myWorker = {
  ping() { return 'pong'; },
  ...
}

Then, AsyncWorker would have an awaitable method that calls it:

const worker = new Worker(new URL(..., import.meta.url), { type: 'module' });
const wrapper = new AsyncWorker(worker, myWorker);
...
await wrapper.ensureReady(/* maybe with an optional timeout? */);

Internally, AsyncWorker.ensureReady() would simply invoke the myWorker.ping() task and would wait on its work item's promise. The return value would be discarded.