Add basic projectiles, enemy hitpoints, enemy death

[benvinegar]

• Add basic projectiles
• Projectiles spawn and move in direction of mouse cursor
• Add collision handler for projectiles
• Add enemy hitpoints, death
• Projectiles spawn outside player
• Give all entities their own layers, projectiles only collide w/ enemies

https://github.com/getsentry/sentaur-survivors/assets/2153/cc80fdc1-86e4-4f93-a202-28e21a51a5cd