Add command mode and allow changing the active mode

[matheusd]

This adds a command mode (":") to the chat app.

In command mode, you can debug (show [model | messages]) or change the active model (model <newmodel>).

More commands are possible in the future.

Messages are preserved when changing the model.

[VictorTaelin]

I don't have GH notifications so I missed this, but can merge once someone fixes the conflicts, sorry