Split environment detection into separate API

[tacotexmex]

Detection of biome types, and cave, water, lava and height detection are all pretty universal features that are useful for more than ambient sound mods.

By detaching the detection feature from the audio control part, the detection part can become a library type of mod.

This also makes it easier to integrate native biome detection support, whenever it gets merged into the engine.

[thePalindrome]

Development for "biome detection" moved to https://github.com/thePalindrome/biome_magic since it'll take magic to figure out where they are :P

Leaving issue open until biome_magic actually does something

[tacotexmex]

minetest.get_biome_id_at_pos is very close to a merge! minetest/minetest#6455 (comment)

[tacotexmex]

The above is now merged into the engine, which should make biome detection trivial.