feat(codegen): Adds support for partial opt-in with already existing properties (#617)

- [X] Fixes an issue with ordering of properties in serialization.
- [X] Adds opt-in with existing properties.

Example:
```cs
private int _myExistingField;

[SerializableField(1)]
public int MyExistingProperty
{
    get => _myExistingField;
    set
    {
        if (value == 0)
        {
            Parent = null;
        }
        
        if (value != _myExistingField)
        {
            ((ISerializable)this).MarkDirty();
            _myExistingField = value;
        }
    }
}
```

Projects/SerializationGenerator/SerializableMigration/SerializablePropertyComparer.cs

@@ -0,0 +1,42 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright 2019-2021 - ModernUO Development Team                       *
+ * Email: hi@modernuo.com                                                *
+ * File: SerializablePropertyComparer.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.Collections.Generic;
+
+namespace SerializationGenerator
+{
+    public class SerializablePropertyComparer : IComparer<SerializableProperty>
+    {
+        public int Compare(SerializableProperty x, SerializableProperty y)
+        {
+            if (Equals(x, y))
+            {
+                return 0;
+            }
+
+            if (Equals(null, y))
+            {
+                return 1;
+            }
+
+            if (Equals(null, x))
+            {
+                return -1;
+            }
+
+            return x.Order.CompareTo(y.Order);
+        }
+    }
+}