Added Abilities

In this update I added the building blocks for implementing player abilities. I also played around with the scripts to add 3 abilities inspired by Hollow Knight's movement : 

1. Wall Slide and Wall Jump : Allows to fall slowly when clinging to walls, then jump in opposite direction
2. Dash : Hollow Knight's core ability. Even though the dash ability only goes horizontally, the code allows for any direction.
3. Multiple Jumps : Players now have a counter of allowed jumps, so abilities only need increase this counter to implement double, triple etc.. jumps.