feat: add listener shortcut to event objects

[LordOfPolls]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

Allows you to add a listener to an event directly from the event class itself. Purely a shortcut method for nicer code.

# before
bot.add_listener(Listener.create("player_update")(self._player_state_update))

# after
PlayerUpdate.listen(self.player_state_update, bot)

Changes

• Add listener classmethod to _BaseEvent

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code