The nyancat mod is not published at Minetest's content database and not in minetest_game,
so a successful installation of the nyanland mod is difficult.
To avoid this dependency difficulty,
the nyancat mod can be embedded into the nyanland mod.
The nyancat mod is already added as a subtree but not yet integrated.
Changes by this commit:
* Execute the embedded nyancat mod code if there is no other nyancat mod installed and modify the mod such that the embedding works
* Disable legacy nyanland and nyancat code by default and add settings to enable them if needed
* Reduce the nyancat:nyancat fall damage
* Some changes to satisfy luacheck
