Implement application commands

[teotwaki]

It doesn't look like Discord is changing its mind when it comes to application commands, so they probably need to be implemented at some point. This is the task I'm currently working on.

I've tried playing around with Poise, but I wasn't a fan of how opinionated it is. That's not meant as a diss or anything, it's just that I'm opinionated as well, and our opinions don't really match. I'm sure that if I had started off building liro using Poise, I would be very happy with it.

I'm taking the overly verbose and declarative route that is currently available in Serenity. I'll see if I get around to publishing a branch with my current WIP, although it may not be super interesting. The main part that I'm trying to figure out is restructuring the existing commands and making that mesh with the application commands, so that I don't need to have two parallel implementations of each command.