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/App.axaml.cs

@@ -0,0 +1,35 @@
+using Avalonia;
+using Avalonia.Controls.ApplicationLifetimes;
+using Avalonia.Data.Core.Plugins;
+using Avalonia.Markup.Xaml;
+using GsaEditor.ViewModels;
+using GsaEditor.Views;
+
+namespace GsaEditor;
+
+public partial class App : Application
+{
+    public override void Initialize()
+    {
+        AvaloniaXamlLoader.Load(this);
+    }
+
+    public override void OnFrameworkInitializationCompleted()
+    {
+        if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
+        {
+            // Avoid duplicate validations from both Avalonia and the CommunityToolkit.
+            var dataValidationPluginsToRemove =
+                BindingPlugins.DataValidators.OfType<DataAnnotationsValidationPlugin>().ToArray();
+            foreach (var plugin in dataValidationPluginsToRemove)
+                BindingPlugins.DataValidators.Remove(plugin);
+
+            desktop.MainWindow = new MainWindow
+            {
+                DataContext = new MainWindowViewModel()
+            };
+        }
+
+        base.OnFrameworkInitializationCompleted();
+    }
+}