Clean up Tilemap

[lishaduck]

Why

There're a lot of unused tiles.

The Plan

#### Tasks
- [ ] Remove tiles w/background.
- [ ] Add more tiles (Maybe a below-slope tile?)
- [ ] https://github.com/PHS-TSA/wizards-in-calculus/issues/34

[lishaduck]

Additionally, we should add a terrain set, which will automatically add the nice edges when placing tiles.