Long running tasks

The `postTask` API is request/response based, and as such doesn't have a natural fit for long running tasks. How do you see such tasks fitting into the model?

For example, let's say I had a worklet which connects to a chat service (via http2 or websocket) and receives chat messages. Currently you could poll the worklet for the next message like so:

```js
const queue = new TaskQueue();

async function run() {
await queue.addModule('/chat-worklet.js');

while(true) {
const task = queue.postTask('nextMessage');
const message = await task.result;
addChatMessageToList(message);
}
}

run();
```

I guess this is ok but it also doesn't seem ideal to me. I think it would make the worklet side more complex in a lot of cases (you might have to keep some global state to know which messages have been sent to which requester).

It would be nice if tasks could also return a stream or something, so that you could use `for/of` to await each message.