Add support for the slnx solution file type

[timozn]

Fixes #1639

Add support for the new .slnx Solution File Format to be recognized as a solution.

[josefpihrt]

@timozn Did you verify that it can load slnx file?

[timozn]

Unfortunately it breaks at a different location now:

I build the NuGet Package and installed from local source:

When running, it cannot open the slnx solution for analysis.

roslynator analyze SwitchNetManager.slnx -o output.file                
Loading solution '/home/timo/Projects/dotnet/switchnetmanager/SwitchNetManager.slnx'...
System.AggregateException: One or more errors occurred. (No file format header found.  /home/timo/Projects/dotnet/switchnetmanager/SwitchNetManager.slnx)
 ---> Microsoft.Build.Exceptions.InvalidProjectFileException: No file format header found.  /home/timo/Projects/dotnet/switchnetmanager/SwitchNetManager.slnx
   at Microsoft.Build.Shared.ProjectFileErrorUtilities.VerifyThrowInvalidProjectFile(Boolean condition, String errorSubCategoryResourceName, BuildEventFileInfo projectFile, Exception innerException, String resourceName, Object[] args)
   at Microsoft.Build.Shared.ProjectFileErrorUtilities.VerifyThrowInvalidProjectFile(Boolean condition, String errorSubCategoryResourceName, BuildEventFileInfo projectFile, String resourceName, Object[] args)
   at Microsoft.Build.Construction.SolutionFile.ParseFileHeader()
   at Microsoft.Build.Construction.SolutionFile.ParseSolution()
   at Microsoft.Build.Construction.SolutionFile.ParseSolutionFile()
   at Microsoft.Build.Construction.SolutionFile.Parse(String solutionFile)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildProjectLoader.LoadSolutionInfoAsync(String solutionFilePath, IProgress`1 progress, ILogger msbuildLogger, CancellationToken cancellationToken)
   at Microsoft.CodeAnalysis.MSBuild.MSBuildWorkspace.OpenSolutionAsync(String solutionFilePath, ILogger msbuildLogger, IProgress`1 progress, CancellationToken cancellationToken)
   at Roslynator.CommandLine.MSBuildWorkspaceCommand`1.OpenProjectOrSolutionAsync(String path, MSBuildWorkspace workspace, IProgress`1 progress, CancellationToken cancellationToken) in /home/timo/Projects/playground/roslynator/src/CommandLine/Commands/MSBuildWorkspaceCommand.cs:line 242
   at Roslynator.CommandLine.MSBuildWorkspaceCommand`1.ExecuteAsync(String path, MSBuildWorkspace workspace, CancellationToken cancellationToken) in /home/timo/Projects/playground/roslynator/src/CommandLine/Commands/MSBuildWorkspaceCommand.cs:line 151
   at Roslynator.CommandLine.MSBuildWorkspaceCommand`1.ExecuteAsync(IEnumerable`1 paths, String msbuildPath, IEnumerable`1 properties) in /home/timo/Projects/playground/roslynator/src/CommandLine/Commands/MSBuildWorkspaceCommand.cs:line 89
   at Roslynator.CommandLine.Program.AnalyzeAsync(AnalyzeCommandLineOptions options) in /home/timo/Projects/playground/roslynator/src/CommandLine/Program.cs:line 343
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at Roslynator.CommandLine.Program.<>c.<Main>b__0_3(MSBuildCommandLineOptions options) in /home/timo/Projects/playground/roslynator/src/CommandLine/Program.cs:line 175
   at CommandLine.ParserResultExtensions.MapResult[T1,T2,TResult](ParserResult`1 result, Func`2 parsedFunc1, Func`2 parsedFunc2, Func`2 notParsedFunc)
   at Roslynator.CommandLine.Program.Main(String[] args) in /home/timo/Projects/playground/roslynator/src/CommandLine/Program.cs:line 169

The Issue seems to be, that msbuild does not yet support to parse slnx files.

There are Tickets that will take care of this: dotnet/msbuild#10794 and dotnet/msbuild#10266

But the planned support for MsBuild 17.13 was revoked and is now planned for 17.14, if I see that correctly.

Thanks for having me checking that first 👍

[josefpihrt]

Ok, please let me know when this MR will be ready to be reviewed again. And thanks for your effort 👍