Tutorial, Caustic Zombie, Player Knockback, and More!

Hey y'all again, I've been cooking up a pretty sizable update this time around with a tutorial stage, new enemy type, player knockback, a bunch of rebalancing and bug fixing!

Added tutorial stage

Added a new enemy type: Caustic

when it dies, it leaves a pool of acid that deals rapid damage to both the player and enemies and ignores iframes

Added knockback to the player whenever they get hit

Added a -15% walking speed penalty while firing the Minigun

Rebalanced some of the early waves in stage 2

The Infected Normal and Parasites now only give 1 point each

Car fire in Parking Cellar no longer gives iframes

Overhauled controls settings menu

Added sounds for the soldier tf2 skin

Fixed the Brute not increasing in speed as its health lowered

Patched an exploit that let the player pretty much fire all the weapons simultaneously

Did some minor UI fixing 

Fixed the god damn health bar being inaccurate (that was so painful...)

Fixed the Boomerang getting thrown when switching to a different weapon and then back in the middle of the throwing animation

Did some other stuff I prob forgor ¯\_(ツ)_/¯