Implement debugitemamount

Implements debugitemamount, which already exists in the game but wasn't hooked up to any code.

ringracers0067

The way this works is that the normal item amount is overridden using cv_kartdebugamount's value as soon as the player "selects" their item, as well as displaying the debug amount in the item roulette drawer.

cv_kartdebugitem.value == item as a condition exists just so the debug amount doesn't bleed into a normal already-set roulette (as shown in the first few seconds of the video), although it will still apply if the item to be debugged already exists in the roulette - not something that super concerns me, but I can tweak it if it's a problem.

Note: 100% sure this is a breaking change and there's no next branch like in Kart v1's repository. If such a branch ends up existing in this repository, let me know and I'll change it.