Powerup overhaul + general changes and bug fixes

In this update, the powerup has been reworked to be a stacking attack speed increase instead of being limited to a 2x firerate for 5 seconds. Picking up a powerup will now start a 5 second timer where any subsequent powerups will increase the firerate factor by 1.

List of changes down below:

• Reworked powerups
• Added a timer so you can see how long your powerup is active for
• Added a HP bar to bosses
• Added a point bonus for each hit you land on the boss
• Fixed an issue where moving diagonally would result in faster movement speed.
• Some code cleaning