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.

Views/MainWindow.axaml.cs

@@ -0,0 +1,11 @@
+using Avalonia.Controls;
+
+namespace GsaViewer.Views;
+
+public partial class MainWindow : Window
+{
+    public MainWindow()
+    {
+        InitializeComponent();
+    }
+}