Add support for .slnx files

[danspark]

Currently, when trying to use dotnet format on a .slnx file, I get the following error:

Unhandled exception: System.IO.FileNotFoundException: The file 'TrafficGenerator.slnx' does not appear to be a valid project or solution file.
   at Microsoft.CodeAnalysis.Tools.Workspaces.MSBuildWorkspaceFinder.FindFile(String workspacePath)
   at Microsoft.CodeAnalysis.Tools.Workspaces.MSBuildWorkspaceFinder.FindWorkspace(String searchDirectory, String workspacePath)
   at Microsoft.CodeAnalysis.Tools.FormatCommandCommon.ParseWorkspaceOptions(ParseResult parseResult, FormatOptions formatOptions)
   at Microsoft.CodeAnalysis.Tools.Commands.RootFormatCommand.FormatCommandDefaultHandler.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken)
   at System.CommandLine.Invocation.InvocationPipeline.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken)

I didn't find any issue mentioning it, so I thought it would be helpful to create this issue.

[ownerer]

Yup, upvoted, I have the same thing.

[ownerer]

I was hoping dotnet/sdk#40913 would fix this, but no such luck.
SDK version 9.0.2 still has this problem.

[sailro]

Now that slnx is fully supported in CLI, it would be great to have support as well in dotnet format. Thanks !

cc @baronfel

[baronfel]

There's a PR to fix this over at dotnet/sdk#47508. Will work with the team to try and get this into the April servicing release.

[JoeRobich]

In addition to the SDK change to allow .slnx files to be loaded, the MSBuildWorkspace which powers dotnet-format solution loading was updated with slnx support in dotnet/roslyn#78647. This has now inserted into the SDK and will be part of .NET 10 preview 6.