Add issues for optional features

Create issues for 

- [ ] options menu
- [x] Level unlocking system
- [ ] Boss behavior and spawning method
- [ ] All mechanics pertaining to infinite mode
- [ ] credits button for main menu
- [ ] Animations for player and enemies
- [ ] Sound design
- [ ] Music
- [ ] The entire card system
- [x] Health decay{testing and implementation}(?)
- [ ] Score mechanic
- [ ] Saving high score