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/SerializableMigrationRulesEngine.cs

@@ -49,6 +49,7 @@ namespace SerializationGenerator
             Compilation compilation,
             string propertyName,
             ISymbol propertyType,
+            int order,
             ImmutableArray<AttributeData> attributes,
             ImmutableArray<INamedTypeSymbol> serializableTypes
         )