Player health and dying

Depends on #1

Pretty self explanatory.

- [x] Enemy attacks should collide with the player (when they intersect the player's grid space) and deal damage to them.
- [x] When the player gets hit, some kind of effect should trigger.
- [ ] When the player dies, reset the scene (or have some kinda game over screen).
- [ ] The player needs to know how much health they have remaining.