Support for .slnx

[PawelGerr]

Serena today only recognizes and parses the legacy .sln format. Microsoft has introduced a new, simpler XML-based solution file format called .slnx that’s now supported by Visual Studio 17.14+ and the .NET SDK 9.0.200+ CLI. As adoption grows, Serena should be able to index, navigate, and manipulate .slnx workspaces just like it does traditional solution files.

Additional context

• New, Simpler Solution File Format – Visual Studio Blog https://devblogs.microsoft.com/visualstudio/new-simpler-solution-file-format/
• Introducing SLNX Support in .NET CLI – .NET Blog https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/
• XML solution parser: Microsoft.VisualStudio.SolutionPersistence

[MischaPanch]

Recently a large PR claiming to improve the C# integration was proposed in #599 . Due to the size of the changes, and since we're not C# developers, we have a hard time reviewing it.

@PawelGerr could you pls check if the issue is solved there? Maybe you could even review some of it or validate that the problems described there actually occur?

[PawelGerr]

The function find_solution_or_project_file currently searches for .sln and .csproj files only. I don't see any changes in #599, so it does not introduce support for .slnx.

Since I'm not a Python developer, I can't say whether the code is more robust or not. However, waiting for #524 to be merged and splitting some of the larger functions into bite-sized ones will definitely make the code easier to review.