Incorporate https://github.com/runceel/ReactiveProperty

#### Feature request

Please, consider bringing-in this amazing library developed by a MSFT Japan employee which is extremely useful on XAML projects like WinUI, WPF, Xamarin, .NET MAUI and so on.

https://github.com/runceel/ReactiveProperty

Please get in touch with the owners (@runceel and @neuecc) of that library and try to bring it to the same umbrella of dotnet/reactive.

I've been using ReactiveProperty on all of my XAML based projects cause if makes my ViewModels more reactive friendly and a lot more readable. But sometimes it's hard to convince big companies to adopt it cause since that's not under the .NET Foundation some folks thinks it doesn't transmit enougth confidence about future maintainability and support on the long run.

IMO I think this will be a greate addition to the set of libraries that Rx team is providing.