Add EntityDeathEvent and implementations

src/main/java/emu/grasscutter/game/entity/EntityMonster.java

@@ -158,6 +158,8 @@ public class EntityMonster extends GameEntity {
 
     @Override
     public void onDeath(int killerId) {
+        super.onDeath(killerId); // Invoke super class's onDeath() method.
+
         if (this.getSpawnEntry() != null) {
             this.getScene().getDeadSpawnedEntities().add(getSpawnEntry());
         }