[ADD] dotnet solution: Migrate

[edvilme]

Created dotnet solution migrate solutionfile.sln to generate a new solution file with the .slnx extension using https://github.com/microsoft/vs-solutionpersistence

Contributes to #40913 (dotnet sln migrate)

[kasperk81]

unit tests upcoming?

Addresses #40913

I thought addressing that epic also includes:

• dotnet new solution --format slnx (with alias -f slnx)
• dotnet sln {add,remove,list} my.slnx my.csproj commands

[edvilme]

unit tests upcoming?

Hello. Yes, this is still WIP and will have a unit test that verifies if the command work. Additional testing on how the conversion is done, is more related to the https://github.com/microsoft/vs-solutionpersistence project as we just consume their API

[kasperk81]

btw, current main and 9.0.200 branch can "build" .slnx #40913 (comment), so the test can assert on that to cover migration.

you may also consider changing Addresses #40913 to Contributes to #40913 so it doesn't close the issue on merge for additional work (dotnet new sln, dotnet sln {add,remove,list} my.slnx my.csproj), and keep this PR for dotnet sln migrate only.