Nyancats are independent in the default mod. Nothing else uses them or
their code. Separating it into a separate mod makes it easier for
subgames to remove them. It also makes it easier for a mod to depend
on nyancats, as lots of subgames don't have them.
Default/mapgen.lua: Register biomes, ores and decorations in
singlenode mapgen. These were never disabled anyway because
singlenode was removed from the world creation menu.
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