Add support for `.slnx` files

[christofjans]

Visual Studio solution files have the .sln file extension. Recently microsoft has been making moves to a new file format: .slnx . There is already support for .slnx in Visual Studio. Terminal-Icons should support it (should probably be same icon as .sln) .

[ardalis]

Came here to suggest the same thing!

[michal-gajda]

        '.slnx'                  = 'nf-dev-visualstudio'

?