Support for worker-to-worker communication

Allow exposing a worker's functionality to other workers using [MessageChannel](https://developer.mozilla.org/en-US/docs/Web/API/MessageChannel).

Workers must handle a new meta message transferring a pair of message ports and listen on it / send to it, too, not just the main thread.

# To Do

- Allow communication using arbitrary MessagePort instances
- Allow passing a MessagePort to spawn() (maybe rather create a new function)
- Add Thread.isTerminatable()
- Export MessageChannel & MessagePort