Sample app updates

[Sergio0694]

✅ Updated all NuGet packages
✅ Switched to System.Text.Json serialization
✅ Enabled nullability annotations in .Core project
✅ Enabled app trimming for .NET Native in UWP sample app

[virzak]

Looking to adapt this library, in a code base which is already annotated with Nullable.
Migrating away from MvvmLight is resulting in:

-        private bool WindowDrop_CanExecute(DragEventArgs e)
+        private bool WindowDrop_CanExecute(DragEventArgs? e)
        {
+            _ = e ?? throw new ArgumentNullException(nameof(e));

everywhere where an argument is used.

Are there any future plans to introduce the API below?

    public sealed class AsyncRelayCommand<T> : ObservableObject, IAsyncRelayCommand<T>, IAsyncRelayCommand, IRelayCommand, ICommand, INotifyPropertyChanged, IRelayCommand<T>
    {
        public AsyncRelayCommand(Func<T?, Task> execute);
        public AsyncRelayCommand(Func<T?, CancellationToken, Task> cancelableExecute);
        public AsyncRelayCommand(Func<T?, Task> execute, Predicate<T?> canExecute);
        public AsyncRelayCommand(Func<T?, CancellationToken, Task> cancelableExecute, Predicate<T?> canExecute);
+        public AsyncRelayCommand(Func<T, Task> execute);
+        public AsyncRelayCommand(Func<T, CancellationToken, Task> cancelableExecute);
+        public AsyncRelayCommand(Func<T, Task> execute, Predicate<T> canExecute);
+        public AsyncRelayCommand(Func<T, CancellationToken, Task> cancelableExecute, Predicate<T> canExecute);

[Sergio0694]

Hi @virzak, adding those overloads would just not be possible, because they'd be conflicting.
As in, T and T? in this context is quite literally the same type, so those overloads are not actually overloads - they're the same.
The reason why the input arguments for these commands is nullable is for consistency with the actual ICommand interface, which takes a nullable object? parameter for both the Execute and CanExecute methods. So we're just following suit with that 🙂

[Sergio0694]

@michael-hawker I've double checked everything's in place and did a force push to keep the history clean.
I've also switched to CommunityToolkit.Mvvm in 6b3a4e9 to make things simpler for #49 👍

[michael-hawker]

Thanks @Sergio0694 I'll try and check the XF sample with this PR today or Monday.

[michael-hawker]

Had to make some small tweaks, but the XF sample is working here too.