Add slnx file migrated from sln

[thegu5]

Both VS and (recently) Rider support the format.

If this is merged now, the .sln file will still have to be relied upon and kept in sync if any changes are made to the .slnx file, since the new format isn't supported everywhere yet. The biggest example is the dotnet CLI, which has release of the feature planned for 9.0 (dotnet/sdk#40913). Rider has a feature to convert between the two.

(Due to the above, maybe hold off on merging?)

🎃