Add Message Listener support

[Zabuzard]

The command system, as of now, only supports SlashCommands. However, we slowly get more and more commands with a need to listen for any incoming message.

The command system should offer a generic way to register message listeners, which should also be combinable with SlashCommands and other things (for example if using the interfaces).

The scope of this issue is only to add message listening support, not other events (such as user banned, message deleted, ...), just message arrived. Maybe also message edit events.

Examples for such commands are the !bytecode command (#207 ) and free-command system (#221 ).

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[Zabuzard]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

This is actually still relevant, we need it eventually.