Fix tab completion for Exp command

[pop4959]

Fixes #3000.

So, the tab completions for the exp command were really wacky in how they worked. Not sure if it was a bug or just poorly designed. I changed it up a bit so that it is slightly more sane, although I didn't rewrite it from the ground up.

Notable changes:

• It correctly suggests completing either player or level now as appropriate.
• When completing level, it will not infinitely ask you to add "l" to the end, and will stop tab completing if you added an "l".
• Tab completing levels now correctly works with the "take" sub command.
• Removed the unnecessary check for "show" when completing for players, because literally every command can take a player as the second argument.
• Many unnecessary branches were removed, which slightly cleaned up the code.

I tested the following tab completions and they all worked after the changes:

/xp <TAB>
/xp give <TAB>
/xp give 1<TAB>
/xp give <player> 1<TAB>
/xp reset <TAB>
/xp set <TAB>
/xp set 1<TAB>
/xp set <player> 1<TAB>
/xp show <TAB>
/xp take <TAB>
/xp take 1<TAB>
/xp take <player> 1<TAB>