Respawning enemies

Depends on #3 and #6

For the MVP, we can just have enemies which infinitely re-spawn. This could ramp up as the player kills enemies or something, or just respawn when an enemy dies.

- [ ] The player should have some kind of tracker for number of enemies killed.
- [ ] Enemies should respawn when the player gets a killed.

Start by reviewing issues #3 and #6, then inspect the existing enemy and player behavior. Done means the player can track enemies killed and enemies respawn after being killed, with the MVP behavior agreed for how respawning should work.

Written by the indexing model from the issue text.