fix(codegen): Adds manual dirty checking (#621)

- Adds `[ManualDirtyChecking]` for anyone that adds it themselves.
- Adds detection of `[Serializable]` and `[ManualDirtyChecking]` at startup to help identify scripts that need migration.

Projects/Server/Items/Item.cs

@@ -783,8 +783,6 @@ namespace Server
             AddNameProperties(list);
         }
 
-        public virtual bool UseDirtyChecking => false;
-
         long ISerializable.SavePosition { get; set; } = -1;
 
         BufferWriter ISerializable.SaveBuffer { get; set; }