Asteroids: Add Alien ship

[NicolenaStiles]

Add occasional alien to spawn in and shoot some shots

[NicolenaStiles]

This is probably gonna go on the backburner with #16.

• [x] graphics
• [ ] component logic
• [ ] game state logic

Note: might have to add a new component, AlienShot, in order to make their shots work. Would also need to rebrand Shot to PlayerShot.