Temporarily set default biome in builtin

This should probably be removed when minetest_game has proper biomes.
If I hear "the whole map is just stone!" again after this, I am going to detonate.

builtin/game/builtin_biome.lua

+minetest.register_biome({
+	name           = "Grass",
+
+	heat_point     = 50.0,
+	humidity_point = 50.0,
+	height_min = -31000,
+	height_max = 31000,
+})
+

builtin/game/init.lua

@@ -25,4 +25,5 @@ dofile(gamepath.."features.lua")
 dofile(gamepath.."voxelarea.lua")
 dofile(gamepath.."forceloading.lua")
 dofile(gamepath.."statbars.lua")
+dofile(gamepath.."builtin_biome.lua")