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/EntitySerializationGenerator.cs

@@ -13,7 +13,6 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
  *************************************************************************/
 
-using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
@@ -64,7 +63,7 @@ namespace SerializationGenerator
             {
                 string classSource = SerializableEntityGeneration.GenerateSerializationPartialClass(
                     kvp.Key,
-                    kvp.Value,
+                    kvp.Value.ToImmutableArray(),
                     context,
                     migrationPath,
                     jsonOptions,