Add alias for simple sub-directory listing

[patridge]

Rather than having to learn anything about the pattern syntax used in GlobbingAliases various methods (including its own GetDirectories method that can be used to similar effect with the right pattern), it seems like it would be nice to have a simplified method for getting a list of all sub-directories in any given DirectoryPath.

Goal syntax:

var directories = GetSubDirectories(someDirectoryPath);

Chat discussion start (approved for issue/PR)

[devlead]

Fixed by #1711