Added the SPAWN_CHANGE event, which occurs when a world's spawn is changed.

This event includes the world who's spawn changed and its previous spawn location.

To listen for this event:
  PluginManager pm = getServer().getPluginManager();
  YourWorldListener worldListener = new YourWorldListener(this);
  pm.registerEvent(Event.Type.SPAWN_CHANGE, worldListener, Priority.Normal, this);

To use this event:
  public class YourWorldListener extends WorldListener {
      @Override
      public void onSpawnChange(SpawnChangeEvent event) {
          World world = event.getWorld();
          Location previousLocation = event.getPreviousLocation();
      }
  }

By: William Bowers <william.bowers@gmail.com>

paper-api/src/main/java/org/bukkit/event/Event.java

@@ -454,6 +454,13 @@ public abstract class Event implements Serializable {
          */
         ITEM_SPAWN (Category.WORLD),
 
+        /**
+         * Called when a World's spawn is changed
+         *
+         * @see org.bukkit.event.world.SpawnChangeEvent
+         */
+        SPAWN_CHANGE (Category.WORLD),
+
         /**
          * Called when a world is saved
          *