Additions
- added power-ups
- new power-up: Screen Wipe
    - clears the screen of enemies
- new sprite the the player when invincible

Changes
- made player a KinematicBody2D and added Area2D as a child of the player for collision detection
    - the game feels the same, but now the movement code is a bit better
- changed the spawn chance for the impostor enemy
- the player can no longer go behind bosses: you simply stop on the side of them
- probably a few other tweaks I forgot about