Added SpawnReason.SLIME_SPLIT. Addresses BUKKIT-790

By: feildmaster <admin@feildmaster.com>

paper-api/src/main/java/org/bukkit/event/entity/CreatureSpawnEvent.java

@@ -139,6 +139,10 @@ public class CreatureSpawnEvent extends EntityEvent implements Cancellable {
          * When an animal breeds to create a child
          */
         BREEDING,
+        /**
+         * When a slime splits
+         */
+        SLIME_SPLIT,
         /**
          * When a creature is manually spawned
          */