Difficulty Overhaul & Many Other Changes

- Added new Elite Orc mob that whirlwinds at the player

- Completely got rid of the Round System and reworked the difficulty into a system that continually gets harder. You know can only purchase and upgrade guns/abilities/hp/mana when the player levels up. Difficulty goes up to 10.0, at which point the player beats that mode.

- Reduced and reworked leveling. Max level is now 12 (will certainly be raised in the future)

- Added two new Abilities: 

         - Orbital Ability will spawn damaging orbs that circle the player, increasing the level increase damage, speed and each level up adds                    another orb; mana cost 50.

         - Time Warp is an AoE slow effect that does not damage enemies, but only costs 25 mana; increasing the level expands the AoE area,                increases length of ability and increases the time that the enemies are slowed for.

- Added special variants of all mobs (including the boss) that appear with a gold outline and do more damage, have more health, are faster and drop treasure chests that when picked up give a random amount of gems. 

- Gems are used at the main menu to upgrade perma upgrades, which persist throughout all playthroughs. Everything else, such as weapon/ability purchases and upgrades reset through each playthrough.

- Added a two new difficulty modes, Heroic and Legendary, which can only be unlocked when the player defeats all enemies at difficulty level 10.0 from the previous difficulty mode.

- Added a save system that saves when the player dies, restarts the game, or finishes difficulty 10.0. The save maintains perma upgrades purchased, gems and difficulty modes unlocked.

- TO RESET YOUR SAVE FILE: go to C:\Users\[USER NAME HERE]\AppData\Roaming\Godot\app_userdata\VSGame and delete the file named "game_save.save"

- Added Shotgun weapon

- Added special effects for some weapons at level 4+:

         - Pistol: bullets home in on the nearest enemy

         - Multi-Shot Gun: additional bullet fires in the middle of the other two

         - Sniper: Slows all enemies hit

         - I have plans to add a carpet bomb effect to the Rocket Launcher and undecided on the special effect for the shotgun as of now

- Many other small bug fixes and changes

- NOTE: There is a weird index error with some collision shape that I can't quite figure out how to debug, but it seems to only happen quite rarely.