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/plugin/java/JavaPluginLoader.java

@@ -379,6 +379,12 @@ public final class JavaPluginLoader implements PluginLoader {
                     ((WorldListener) listener).onChunkUnload((ChunkUnloadEvent) event);
                 }
             };
+        case SPAWN_CHANGE:
+            return new EventExecutor() {
+                public void execute(Listener listener, Event event) {
+                    ((WorldListener) listener).onSpawnChange((SpawnChangeEvent) event);
+                }
+            };
         case WORLD_SAVE:
             return new EventExecutor() {
                 public void execute(Listener listener, Event event) {