Migrate to .slnx solution file format (#82)

* Initial plan

* Initial plan for dotnet sln migrate

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

* Add .slnx file using dotnet sln migrate

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

* Delete :GITHUB_ENV

* Remove .sln file, keep only .slnx for CI/CD compatibility

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>
Co-authored-by: Benjamin Michaelis <benjamin@michaelis.net>

Author: Copilot

IntelliTect.TestTools.Console.sln

@@ -0,0 +1,10 @@
+<Solution>
+  <Folder Name="/Solution Items/">
+    <File Path=".github/workflows/build-and-test.yml" />
+    <File Path=".github/workflows/deploy.yml" />
+    <File Path="Directory.Build.props" />
+    <File Path="global.json" />
+  </Folder>
+  <Project Path="IntelliTect.TestTools.Console.Tests/IntelliTect.TestTools.Console.Tests.csproj" />
+  <Project Path="IntelliTect.TestTools.Console/IntelliTect.TestTools.Console.csproj" />
+</Solution>