Improved Movement Scripts
work on #48
- Reworks the various movement scripts to use PhysicsMovement and MoveTowardsTarget as base classes to cut down on code duplications.
- Where appropriate, changes Start() methods to be OnEnable() so that movement scripts can be used as states for the bosses.
- Adds physics layers for Player Projectile and Boss Projectile, and corrects the physics layers on some of the existing prefabs.
    - This fixes the issue where the boss couldn't shoot the player if they were too close.