[FEAT] Improvements to message commands

[AstreaTSS]

Is your feature request related to a problem? Please describe.
Message commands are limited, honestly. They have many problems, least of which is the fact that multiple arguments with differing types is a pain and subcommands are basically not happening with them.

Describe the solution you'd like
A system very much like discord.py's message commands: a nice, mostly sane system to do everything a person would need.

@message_command()
async def command(self, ctx, num: int, user: User):
    ...

Describe alternatives you've considered
Doing it yourself with the command arguments, which isn't pleasant, speaking from experience.
It's about about time we pointed out molter exists, but molter will have to do more and more hacks to get things working as time goes on, and not every new user will know what molter is.

Additional information
See molter's #3 for what needs to be done to merge molter here, which would solve the issue.

Under no circumstances should this issue hold back the beta release. Polls and I have agreed it will not count towards it. It is under the milestone anyways to place a soft-soft deadline on myself.

[AstreaTSS]

Gonna add a note: this would be the perfect time to switch to the "prefixed commands" terminology that has been in use more and more frequently in the Discord bot development space. It helps separate them from context menu message commands while still being mostly clear.

[AstreaTSS]

Closed by #444.