sln-remove: 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 remove support for .slnx files

Breaking changes

• Command will fail when passed a solution file with duplicate sources: vs-solutionpersistence does not allow duplicate references (ie, listing the same project twice) in a solution file.

[kasperk81]

hoist childrenByParent computation out of recursive step

[kasperk81]

@edvilme lets address the feedback and get this in 9.0.200 to close the lid on this feature. without this, IDEs like vscode which rely on sdk aren't going to use this feature

[baronfel]

@kasperk81 he's out for vacation - this command will still make 9.0.200 (code freeze is mid-January) but likely won't make a particular VS preview insertion as a result.