Add can_spawn callback in mob api

mods/ENTITIES/mcl_mobs/api.lua

@@ -4034,6 +4034,7 @@ minetest.register_entity(name, {
 	teleport = teleport,
 	do_teleport = def.do_teleport,
 	spawn_class = def.spawn_class,
+	can_spawn = def.can_spawn,
 	ignores_nametag = def.ignores_nametag or false,
 	rain_damage = def.rain_damage or 0,
 	glow = def.glow,

mods/ENTITIES/mcl_mobs/spawning.lua

@@ -533,6 +533,9 @@ if mobs_spawn then
 							return
 						end
 					end
+					if minetest.registered_entities[mob_def.name].can_spawn and not minetest.registered_entities[mob_def.name].can_spawn(pos) then
+						return
+					end
 					--everything is correct, spawn mob
 					local object
 					if spawn_in_group then