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/Server/Serialization/SerializablePropertyAttribute.cs

@@ -1,30 +0,0 @@
-/*************************************************************************
- * ModernUO                                                              *
- * Copyright (C) 2019-2021 - ModernUO Development Team                   *
- * Email: hi@modernuo.com                                                *
- * File: SerializablePropertyAttribute.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>
-    /// Marks a property as serializable. Requires a call to ISerializable.MarkDirty()
-    /// </summary>
-    [AttributeUsage(AttributeTargets.Property)]
-    public sealed class SerializablePropertyAttribute : Attribute
-    {
-        public int Order { get; }
-
-        public SerializablePropertyAttribute(int order) => Order = order;
-    }
-}