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/SerializableFieldAttribute.cs

@@ -17,7 +17,12 @@ using System;
 
 namespace Server
 {
-    [AttributeUsage(AttributeTargets.Field)]
+    /// <summary>
+    /// Hints to the source generator that this field or property should be serialized.
+    /// When used on a field, the source generator will generate the property entirely.
+    /// When used on a property, the user must call ((ISerializable)this).MarkDirty().
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
     public sealed class SerializableFieldAttribute : Attribute
     {
         public int Order { get; }