[voxel-flatland] Flat land generator, simple terrain generation example

[deathcap]

A very simple terrain plugin which only generates flat land, no other features. This would serve as a trivial xample.

Similar to https://github.com/voxel/voxel-land but simplified extensively (voxel-land itself may also want to have an optional flat land generator, with other terrain generation options e.g. trees, ponds, etc. but this hypothetical flat land plugin would only do flat land).

https://github.com/deathcap/voxel-example could use this generator instead of bundling its own in lib/terrain.js (and lib/blocks.js), and it could be useful in other applications which only need simple terrain.

[deathcap]

https://github.com/voxel/voxel-flatland