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/SerializableFieldAttributeAttribute.cs

@@ -17,6 +17,12 @@ using System;
 
 namespace Server
 {
+    /// <summary>
+    /// Hints to the source generator that this field will need this attribute on the generated property
+    /// [SerializableFieldAttr("[CommandProperty(AccessLevel.GameMaster)]")]
+    /// -or-
+    /// [SerializableFieldAttr(typeof(CommandPropertyAttribute), AccessLevel.GameMaster)]
+    /// </summary>
     [AttributeUsage(AttributeTargets.Field)]
     public sealed class SerializableFieldAttrAttribute : Attribute
     {