Add PlaceholderAPI support for showing world load status (%multiverse-core_isloaded_<worldname>%)

Author: benwoo1110

src/main/java/org/mvplugins/multiverse/core/PlaceholderExpansionHook.java

@@ -150,6 +150,7 @@ public boolean persist() {
             case "gamemode" -> world.getGameMode().toString().toLowerCase();
             case "generator" -> world.getGenerator();
             case "hunger" -> String.valueOf(world.isHunger());
+            case "isloaded" -> String.valueOf(world.isLoaded());
             case "monstersspawn" -> String.valueOf(world.getEntitySpawnConfig()
                     .getSpawnCategoryConfig(SpawnCategory.MONSTER)
                     .isSpawn());