Add project restore files and NuGet cache for GsaViewer

- Created project.nuget.cache to store NuGet package cache information.
- Added project.packagespec.json to define project restore settings and dependencies.
- Included rider.project.restore.info for Rider IDE integration.

ViewModels/MainWindowViewModel.cs

@@ -0,0 +1,6 @@
+namespace GsaViewer.ViewModels;
+
+public partial class MainWindowViewModel : ViewModelBase
+{
+    public string Greeting { get; } = "Welcome to Avalonia!";
+}