Add JSON user settings

[Ruben2776]

#58

~~Tutorial at https://makolyte.com/csharp-how-to-read-custom-configuration-from-appsettings-json/~~

Try to do data persistence https://docs.avaloniaui.net/guides/deep-dives/reactiveui/data-persistence || https://github.com/reactiveui/ReactiveUI.Samples/tree/main/avalonia/suspension/ReactiveUI.Samples.Suspension