Add selection logic and path finding

[nafeu]

https://user-images.githubusercontent.com/6453335/196023935-36d8da90-84e4-4a21-89fe-5a66b25573cc.mov

Notes:

• Use shift + click to spawn Units
• Use click to select a unit
• Use rightClick to move unit to a location

Changes:

• Add basic isMoving state
• Add selected state stored in a new entity
• Rename Turret to Unit
• Implement basic path finding