[Bleeding] Add new TargetReasons to EntityTargetEvent.

This commit adds three new TargetReasons to EntityTargetEvent to address
missing cases where the event is not currently fired.

The first, TargetReason.TARGET_ATTACKED_NEARBY_ENTITY, is used when a
neutral wolf is attacked, causing all nearby wolves to turn hostile and
attack the first wolf's target.

The second, TargetReason.REINFORCEMENT_TARGET, is used when a zombie summons
reinforcements and the new zombie targets the first zombie's target.

The third, TargetReason.COLLISION, is used when an iron golem collides with
a hostile entity, causing it to begin targeting the entity it collided with.

By: GJ <gjmcferrin@gmail.com>

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

@@ -121,6 +121,18 @@ public class EntityTargetEvent extends EntityEvent implements Cancellable {
          * When an entity selects a target while defending a village.
          */
         DEFEND_VILLAGE,
+        /**
+         * When the target attacks a nearby entity of the same type, so the entity targets it
+         */
+        TARGET_ATTACKED_NEARBY_ENTITY,
+        /**
+         * When a zombie targeting an entity summons reinforcements, so the reinforcements target the same entity
+         */
+        REINFORCEMENT_TARGET,
+        /**
+         * When an entity targets another entity after colliding with it.
+         */
+        COLLISION,
         /**
          * For custom calls to the event.
          */