Core/AI: Add a new LeavingWorld AI hook

Port From (https://github.com/TrinityCore/TrinityCore/commit/2642fb1a489e4b79a17bbb63127c8b3ce2823e1e)

Source/Game/AI/CoreAI/UnitAI.cs

@@ -480,6 +480,10 @@ namespace Game.AI
         // Called when the unit leaves combat
         public virtual void JustExitedCombat() { }
 
+        // Called when the unit is about to be removed from the world (despawn, grid unload, corpse disappearing, player logging out etc.)
+        public virtual void LeavingWorld() { }
+
+        // Called at any Damage to any victim (before damage apply)
         public virtual void DamageDealt(Unit victim, ref uint damage, DamageEffectType damageType) { }
         public virtual void DamageTaken(Unit attacker, ref uint damage) { }
         public virtual void HealReceived(Unit by, uint addhealth) { }

Source/Game/Entities/Unit/Unit.cs

@@ -435,6 +435,9 @@ namespace Game.Entities
             if (IsInWorld)
             {
                 m_duringRemoveFromWorld = true;
+                if (IsAIEnabled)
+                    GetAI().LeavingWorld();
+
                 if (IsVehicle())
                     RemoveVehicleKit(true);