Add Permafrost as new dirt variant in ice caves

Dirt is normally prevalent in caves and now would be a great chance to add Permafrost, a frozen dirt variant, to the game! It can form in blobs throughout the caves. It would act like Podzol in that grass and mycelium cannot grow on it. This would add a nice new dirt variant that would be great for cold biomes. Currently, we don't really have any good dirt variants for cold biomes aside from podzol and the snowy grass block, neither of which look particularly different from normal dirt when covered in snow. Permafrost could even be added to existing biomes like the tundra in splotches to give some variance to the kinds of dirt there.