Refactor command structure

[maybe-yiyi]

Currently, the command structure has a main command and subcommands, e.g. (/courses has unlocks and course-info).
We want to move these into separate files, to reduce file size and command typing (less characters to type). However, we still want to keep the files organized.

Things to do:

• Move all the subcommands into their own files, in a grouped folder. For example, there will be a src/commands/courses folder that contains an unlocks.ts, course-info.ts, fce.ts, etc.
• Move all shared functions in to a utils.ts for that folder. For example, we would move the Course type into src/commands/courses/utils.ts
• Rewrite the command handler to match this new structure
• Also redirect commands should be done like this