Allow posting messages to web workers

[chemicstry]

Currently it's not possible to send/receive messages to/from web workers because library overtakes on_message callbacks.

It would be nice to somehow allow sending custom messages over API. It might also be possible to release on_message callbacks aftrer initialization and use channels for communication inside the library.

This feature would probably need a major rewrite as discussed in https://github.com/chemicstry/wasm_thread/issues/8