Integrate with Discord slash command system

[jgcodes2020]

As per title. This would make it simpler for new users to use (for example, $encode), and offers some extra features that standard message listeners don't have.

[MikeXander]

This requires completely rewriting the entire codebase because a lot of argument parsing isn't always at the start of every command. Personally, I will do this for the encode command and nothing else. Happy if others contribute