Basic enemy scene with player tracking

[Sdc97]

Issue

Fixes #12

Notes

• Implements a basic enemy scene
  • Collision box on enemy
  • Moves between between a spawn point and a marker
  • "Aggro" radius to detect player
  • Slime will follow player while player remains in radius
  • Up and down animations for movement

Testing

• Basic enemy wandering
• Enemy follows player when within range, collides with physics enabled terrain