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.

首先查看 issue #3 和 #6，然后检查现有的敌人和玩家行为。当玩家能够跟踪已击杀的敌人，并且敌人在被击杀后会重生，同时已就重生应如何运作达成 MVP 行为共识时，即表示完成。

由索引模型根据 Issue 内容生成。