Biome API: Add 'get_biome_name(biome_id)' API

[paramat]

For #7105
Many APIs use the biome ID, this is useful to get the name string from the ID.

Tested with this mod that prints biome name as you explore:

local timer = 0

	minetest.register_globalstep(function(dtime)
		timer = timer + dtime
		if timer > 2 then
			timer = 0
			for _, player in ipairs(minetest.get_connected_players()) do
				local pos = player:getpos()
				pos.x = math.floor(pos.x)
				pos.y = math.floor(pos.y)
				pos.z = math.floor(pos.z)
				local biome_data = minetest.get_biome_data(pos)
				local biome_name = minetest.get_biome_name(biome_data.biome)
				print(biome_name)
			end
		end
	end)

Note in mgv6 which does not define biomes 'default' is returned.

[paramat]

Docs added, mergeable.

[paramat]

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