Improve application commands

[NeloBlivion]

• Add easily-accessible methods to update and create commands manually
• Consider saner syncing logic

Potentially depends on #50

[Paillat-dev]

Also depends on cogs rewrite most likely or at least to some extent. Would love to chat with you in detail on the exact implementation

[NeloBlivion]

It MIGHT be the other way round; this is addressing base implementations that currently don't exist such as SlashCommand.edit, which then rolls into improving syncing which cog command loading is heavily tied into. But indeed, would also love to discuss.

[Paillat-dev]

Actually, you're probably right. Also don't bother with ext.commands

[Paillat-dev]

I am going to use this issue to hold application commands / slash command related issue if you don't mind, also since you mentioned auto-syncing I added the auto syncing issue in here directly