Renamed WebWorker to MessagePort

The WebWorker adapters were always implemented to the MessagePort spec.
The renaming is an acknowledgment that other types implement the
MessagePort API. There is no functional change.

- `msgs/adapters/webWorker`      -> `msgs/adapters/messagePort`
- bus.inboundWebWorkerAdapter()  -> bus.inboundMessagePortAdapter()
- bus.outboundWebWorkerAdapter() -> bus.outboundMessagePortAdapter()
- bus.webWorkerGateway()         -> bus.messagePortGateway()