add platformer legnth

Author: Folleach

GeometryDashAPI/Server/Dtos/LevelPreview.cs

@@ -37,7 +37,7 @@ public string Description
 
         [GameProperty("6")]
         public int AuthorUserId { get; set; }
-        
+
         [GameProperty("8")]
         public Difficulty Difficulty { get; set; }
 

GeometryDashAPI/Server/Enums/LengthType.cs

@@ -6,6 +6,7 @@ public enum LengthType
         Short = 1,
         Medium = 2,
         Long = 3,
-        XL = 4
+        XL = 4,
+        Platformer
     }
 }