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.

GsaEditor/ViewModels/ViewModelBase.cs

@@ -0,0 +1,7 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace GsaEditor.ViewModels;
+
+public class ViewModelBase : ObservableObject
+{
+}