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/GsaEditor.csproj

@@ -0,0 +1,31 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>WinExe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <BuiltInComInteropSupport>true</BuiltInComInteropSupport>
+    <ApplicationManifest>app.manifest</ApplicationManifest>
+    <AvaloniaUseCompiledBindingsByDefault>true</AvaloniaUseCompiledBindingsByDefault>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <AvaloniaResource Include="Assets\**"/>
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Avalonia" Version="11.3.6"/>
+    <PackageReference Include="Avalonia.Desktop" Version="11.3.6"/>
+    <PackageReference Include="Avalonia.Themes.Fluent" Version="11.3.6"/>
+    <PackageReference Include="Avalonia.Fonts.Inter" Version="11.3.6"/>
+    <PackageReference Include="Avalonia.Diagnostics" Version="11.3.6">
+      <IncludeAssets Condition="'$(Configuration)' != 'Debug'">None</IncludeAssets>
+      <PrivateAssets Condition="'$(Configuration)' != 'Debug'">All</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.2.1"/>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\GsaEditor.Core\GsaEditor.Core.csproj"/>
+  </ItemGroup>
+</Project>