Drop hardcoded node names

[Wuzzy]

The mod currently has some hardcoded node names in it. This essentially makes the mod hard-depend on default. Not all games which have dirt, stone and dirt-with-grass also do add them in a mod named default. So this mod will (avoidably) fail in all games without default.

This is avoidable.

I suggest to drop the hardcoded node names. Instead, read them from mapgen aliases (minetest.registered_aliases), i.e. mapgen_dirt, mapgen_stone and mapgen_dirt_with_grass, then resolve them to their real node names.

Only if these aliases are empty or non-resolvable, fall back to the default nodes and (maybe) write an error or warning message on mod start.