Added UV for generated walls

I working on an open source life simulation game with the working title Project Harmonia using Bevy.

It was an interesting task and I learned a lot about how meshes are represented internally.

Initially I used 8 vertices for each wall. But then I discovered that if I needed a different color and normal for each side, then I needed a different vertex! So I reworked the mesh generation to make 20 vertices per mesh (for each side with no bottom).