12 add single player basic ai AND 13 complete simple game loop

[mitchellkazin15]

Sorry, lots of changes here... Got a little carried away but I also had to add a lot of infrastructure before I could even develop a simple AI

Major Changes:

• Spawn Multiple arenas
• Programmatically spawn main player and computer players
• Add movement AI to computer players
  • AI has 3 competing objectives
    1. Move opposite direction of closest enemy
    2. Try to head towards the center
    3. keep heading in a similar direction (looks less jittery and less prone to getting stuck between 2 close enemies)
  • Weight of movement objectives shifts based on how close enemies are or how far it is from the center
• Complete Simple game loop
  • If all players in a given arena die, the other team wins

Minor Changes:

• Add number selector button type
• Group players an enemies by arena name to keep targeting separate
• Added input components for movement and upgrade selection
  • This allows more code to be reused between main and computer players
• Changed back to move_and_slide for players and enemies
  • had to change motion_mode to "floating" to avoid snapping to other players/enemies
• Other minor bug fixes/refactoring