feat(home-manager): backup linked files once if they already exist during activation

[rrvsh]

Please do one of the following:

• In standalone mode, use 'home-manager switch -b backup' to back up files automatically.
• When used as a NixOS or nix-darwin module, set either
  • 'home-manager.backupFileExtension', or
  • 'home-manager.backupCommand',
    to move the file to a new location in the same directory, or run a custom command.
• Set 'force = true' on the related file options to forcefully overwrite the files below. eg. 'xdg.configFile."mimeapps.list".force = true'
  Existing file '/Users/rafiq/.codex/config.toml' would be clobbered
  Error:
  0: Darwin activation failed
  1: Activating configuration (