[Exiled::API] Add ParentCheckpointDoor (Exiled-Team#2651)

* Add `ParentCheckpointDoor`

* ParentCheckpointDoor in ctor

Exiled.API/Features/Doors/Door.cs

@@ -61,6 +61,8 @@ internal Door(DoorVariant door, List<Room> rooms)
             if (Type is DoorType.Unknown)
                 Log.Error($"[DOORTYPE UNKNOWN] {this}");
 #endif
+
+            ParentCheckpointDoor = Get(Base.GetComponentInParent<Interactables.Interobjects.CheckpointDoor>());
         }
 
         /// <summary>
@@ -159,7 +161,12 @@ public bool IsClosed
         /// <summary>
         /// Gets a value indicating whether or not this door is subdoor belonging to a checkpoint.
         /// </summary>
-        public bool IsPartOfCheckpoint => List.Any(x => x is Checkpoint checkpoint && checkpoint.Subdoors.Contains(this));
+        public bool IsPartOfCheckpoint => ParentCheckpointDoor is not null;
+
+        /// <summary>
+        /// Gets the checkpoint door the door is belonging to, or null if the door doesn't belong to a checkpoint.
+        /// </summary>
+        public Door ParentCheckpointDoor { get; }
 
         /// <summary>
         /// Gets a value indicating whether or not this door requires a keycard to open.