[WIP] Use new parser from vs-solutionpersistence to later enable .slnx support

[mruxmohan4]

Kept as draft for ease of review.

[kasperk81]

slngen's command line options are

  -d|--solutiondir <path>             An optional path to the directory in which the solution file will be generated. Defaults to the same directory as the project. --solutionfile will take precedence over this switch.
  -o|--solutionfile <path>            An optional path to the solution file to generate. Defaults to the same directory as the project.

consider adding --format <sln|slnx>

upcoming dotnet SDK 9.0.2xx (and v10 from daily builds) has

dotnet new sln --help
...
Template options:
  -f, --format <sln|slnx>  Choose the format for the solution file: sln or slnx.
                           Type: choice
                             sln   Solution file
                             slnx  XML Solution file
                           Default: sln

see the feedback from VS team about naming/terminology dotnet/sdk#44469 (comment)