tile_prototype.next_upgrade

[Mylon]

I'd like tile prototypes to support a next_upgrade property.
No mechanical effect. Just something that could be read in script. So "stone-path" would have "concrete" and "concrete" would have "refined-concrete" and "hazard-concrete-left" would have "refined-hazard-concrete-left", and so on.

This would make it easier for my mod Concreep to work with mods that add more tile types, like Space Exploration's scaffold vs plating.