Add get_biome_id(biome_name) callback

It returns the index used in mg->biomemap for a given biome name.
The biomemap is useless without this unless you re-register all existing biomes,
which could cause problems for anyone else trying to use biomemap.
With this, you can quickly create a lookup table of ids and names.

src/script/lua_api/l_mapgen.h

@@ -24,6 +24,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 class ModApiMapgen : public ModApiBase {
 private:
+	// get_biome_id(biomename)
+	// returns the biome id used in biomemap
+	static int l_get_biome_id(lua_State *L);
+
 	// get_mapgen_object(objectname)
 	// returns the requested object used during map generation
 	static int l_get_mapgen_object(lua_State *L);