[Open Assets] Custom Boost 1.5.1

It's been a while, but recently I've been playing SRB2 Generations, and one bug in particular really bothered me, you could use the boost during the stage select screen. This version is a simple fix for that, and will probably be the last version of this script, since there seems to be some better stuff coming up .

===Changelog===

• Fixed a glitch where you wouldn't be able to boost after respawning;
• Cleaned up some of the code to minimize redundancy.

Changelog​
Air Boost Added proper Air Boost mechanics inspired by Sonic Forces.
Amy and Fang fixed! Fixed Boost not working properly with Amy and Fang

Minor refactoring and bug fixes.

Spoiler: CHANGELOG

• Renamed local functions, removed those P_ prefixes I added for no reason;
• Fixed a few bugs in multiplayer caused by having MAX_BOOST and PREVSKIN as local script variables instead of player properties;
• Removed a stupid comment I added about not needing to give credit;
• Added split-screen commands.