Changes

`/ex(s)`: remove redundant checks, minor cleanups

Checks such as checking the command name, permission, etc. are no longer needed, as this is handled by Bukkit internally.
Also few minor cleanups:
- Updated some code to `instanceof` pattern matching
- Cleaned up sending debug to the player; can just use `sender`'s `sendMessage` methods instead of casting it to a player
- Renamed `onTabComplete`'s `cmdName` param to `alias`, as that's what it actually is.
- Removed some old commented out code from `ExCommandHandler`.

Reported on [Discord](https://discord.com/channels/315163488085475337/1133477089606123640).
 (commit: 014df1d)