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/SerializableProperty.cs

@@ -17,7 +17,7 @@ using System.Text.Json.Serialization;
 
 namespace SerializationGenerator
 {
-    public class SerializableProperty
+    public record SerializableProperty
     {
         [JsonPropertyName("name")]
         public string Name { get; init; }
@@ -30,5 +30,8 @@ namespace SerializationGenerator
 
         [JsonPropertyName("ruleArguments")]
         public string[] RuleArguments { get; init; }
+
+        [JsonIgnore]
+        public int Order { get; init; }
     }
 }