Command registration rewrite (#759)

* Initial commit for command registration rewrite

* Fix faulty check in registration

* Fix logic in command registration

* Document get_desynced_commands

* Fix logic in registration and update docstring

* Fix error in docs

* Finish application command registration

* Prep for beta 2

Co-authored-by: Lala Sabathil <lala@pycord.dev>

Author: BobDotCom