Rewrite `ext.commands` and somewhat refactor

[Paillat-dev]

Extract ext.commands so that it is, at minima entirely separated from the main module, and at maxima a different library. Additionally rewrite some of its mess and simplify it.

This implies that ext.commands should not use ANY private method from the base py-cord, not use ANY private util or anything apparent to that. It should exclusively sit on top of py-cord's event system.

ext.commands is removed in #217

[Lumabots]

for the ext command design, do we want something closer to our next app commands ? or should we keep them distinct like rn.
Be similar could help as it will be easier to go from one to another, while being fully distinct could. help us reach the full ability of prefix

[Lumabots]

and for bridge, should we keep our bot subclass or just add a function to convert command to bridge.
not subclass bot will lead to the loss of bot.bridge_commands, on_bridge_command event etc