Voxel demo

Demo of the voxel library I'm working on, using Ebitengine.
`WASD` to move, `space` to jump, `shift` to run, `mouse wheel` to pick another block, `left/right click` to add/gather blocks.
`1,2,3,4` also allow you to pick a different block to add in the world.

Collisions are not perfect if shift-running