SPIGOT-3102: Add EXPLOSION SpawnReason

By: md_5 <git@md-5.net>

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

@@ -157,6 +157,10 @@ public class CreatureSpawnEvent extends EntitySpawnEvent {
          * When an cow is spawned by shearing a mushroom cow
          */
         SHEARED,
+        /**
+         * When eg an effect cloud is spawned as a result of a creeper exploding
+         */
+        EXPLOSION,
         /**
          * When a creature is spawned by plugins
          */