sln-list: Support for slnx

[edvilme]

Contributes to #40913

The dotnet CLI should support the new slnx format for building and in the existing solution management commands. It should also help interested users migrate to the new format.

This adds dotnet sln list support for .slnx files

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This also adds reusable logic to SlnCommandParser to be reused by other sln commands moving forward.

[kasperk81]

@baronfel this looks good?

[mthalman]

This broke in internal CI: https://dev.azure.com/dnceng/internal/_build/results?buildId=2575716&view=results (internal link). It looks like it got merged w/o syncing in the changes from #43015 that were merged before this one. They don't work together.

[kasperk81]

#44661

[nagilson]

Late to review but this looks great! I may have tweaked the wording a little bit on the command strings, but that is a nit; I dont think its worth doing that now. Love the use of the Range Operator :)