Pack Zombie basics done.

Will still have to handle turret targeting.

Otherwise, pack zombie behaves as expected: They will wander around
until they see a leader or Zed. They will chase Zed if they see him.
Otherwise, they will assume a position around the leader. When the
leader sees Zed, the Pack zombies will chase Zed down.

ZombieTargetSelector is scripted in preparation for Turret target
selection.