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/Serialization/ManualDirtyChecking.cs

@@ -0,0 +1,28 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: ManualDirtyChecking.cs                                          *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+
+namespace Server
+{
+    /// <summary>
+    /// Indicates that the applied class has dirty checking. This is necessary for classes that are not code genned.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class)]
+    public class ManualDirtyCheckingAttribute : Attribute
+    {
+
+    }
+}