[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.