A more flexible handler API and supervisor-friendly start function

[vshakitskiy]

#20

Didn't do any test cases related to WebSocket restarts, uses of stop or stop abnormal yet. Technically everything should work. I added some new functions and types to root module, and also adjusted example bot to have start for handler with state and custom message. You can try things out switching simple handler and "normal" handler. Don't really like the names, so maybe you have better ideas :D

[vshakitskiy]

• On discord_gleam.stop, event loop actor will still remain alive, meaning it wont be restarted by supervision tree, while discord websocket connection will be closed, as we want it to be closed.

• On discord_gleam.stop_abnormal, actor will also stop abnormal. Since in example bot our actor is being part of supervision tree, actor with websocket connection is being successfully restarted.

I guess next functions works like I intended, but I still don't know about how correct does websocket reconnection logic work inside event loop actor

[cyteon]

Testing this tmrw then merging