Fix followers getting sent to nullspace when target is polymorphed (#33878)

* Add an event for polymorph actions

* Subscribe FollowerSystem to the event

* Add check that unfollow target is actually current target.

* Make followers try to follow on state update; Dirty follower on follow.

Content.Shared/Polymorph/PolymorphEvents.cs

@@ -0,0 +1,10 @@
+namespace Content.Shared.Polymorph;
+
+/// <summary>
+/// Raised locally on an entity when it polymorphs into another entity
+/// </summary>
+/// <param name="OldEntity">EntityUid of the entity before the polymorph</param>
+/// <param name="NewEntity">EntityUid of the entity after the polymorph</param>
+/// <param name="IsRevert">Whether this polymorph event was a revert back to the original entity</param>
+[ByRefEvent]
+public record struct PolymorphedEvent(EntityUid OldEntity, EntityUid NewEntity, bool IsRevert);