Add support for XML-based solution file format (`slnx`)

[egvijayanand]

• Add support for XML-based solution file format (slnx)
• Should be an opt-in as the slnx feature itself is still in the preview phase

dotnet new winforms-blazor -o MyApp -slnx

dotnet new wpf-blazor -o MyApp -slnx

[egvijayanand]

This feature is now available as part of the below releases.

• VijayAnand.BlazorTemplates - v4.3.0 (Unified package)
• VijayAnand.WindowsForms.Blazor - v4.3.0
• VijayAnand.Wpf.Blazor - v4.3.0
• Blazor Desktop Templates - v4.3.0.0