Add .slnx file using dotnet sln migrate

Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

EssentialCSharp.ListingManager.slnx

@@ -0,0 +1,15 @@
+<Solution>
+  <Configurations>
+    <Platform Name="Any CPU" />
+    <Platform Name="x64" />
+    <Platform Name="x86" />
+  </Configurations>
+  <Folder Name="/Solution Items/">
+    <File Path="Directory.Build.props" />
+    <File Path="Directory.Packages.props" />
+    <File Path="NuGet.config" />
+    <File Path="README.md" />
+  </Folder>
+  <Project Path="ListingManager.Tests/EssentialCSharp.ListingManager.Tests.csproj" />
+  <Project Path="ListingManager/EssentialCSharp.ListingManager.csproj" />
+</Solution>