[folderutil] prioritize "$HOME/.config" on Darwin

[robsonpeixoto]

Prioritizing the $HOME/.config on Darwin make easier share configuration between machines, and other UNIX OS, using some dotfile tools.

[dogancanbakir]

Thanks for the suggestion, the dotfile use case makes sense! folderutil is shared by all our tools, and we moved to os.UserConfigDir() for a stable, standard per-OS location. Switching the macOS default to ~/.config would be breaking for existing installs, , so this needs care.

cc @Mzack9999 for thoughts.