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/PreviewMode.cs

@@ -0,0 +1,12 @@
+namespace GsaEditor.ViewModels;
+
+/// <summary>
+/// Determines which preview panel to show for the selected archive entry.
+/// </summary>
+public enum PreviewMode
+{
+    None,
+    Text,
+    Image,
+    HexDump
+}