Core/AreaTrigger: Implement AreaTriggerActionSetFlag::DontDespawnWithCreator

Port From (https://github.com/TrinityCore/TrinityCore/commit/c40b6e0b3f1882a9fce8dfd103be4291ca986851)

Source/Framework/Constants/UnitConst.cs

@@ -616,4 +616,10 @@ namespace Framework.Constants
         CanSkin = 0x40,     // previously UNIT_DYNFLAG_DEAD
         ReferAFriend = 0x80
     }
+
+    public enum AreaTriggerRemoveReason
+    {
+        Default,
+        UnitDespawn
+    }
 }

Source/Game/Entities/Unit/Unit.cs

@@ -866,10 +866,15 @@ namespace Game.Entities
             }
         }
 
-        public void RemoveAllAreaTriggers()
+        public void RemoveAllAreaTriggers(AreaTriggerRemoveReason reason = AreaTriggerRemoveReason.Default)
         {
-            while (!m_areaTrigger.Empty())
-                m_areaTrigger.Last()?.Remove();
+            foreach (AreaTrigger at in m_areaTrigger)
+            {
+                if (reason == AreaTriggerRemoveReason.UnitDespawn && at.GetTemplate().ActionSetFlags.HasFlag(AreaTriggerActionSetFlag.DontDespawnWithCreator))
+                    continue;
+
+                at.Remove();
+            }
         }
 
         public NPCFlags GetNpcFlags() { return (NPCFlags)m_unitData.NpcFlags.GetValue(); }