Stabilise nix store delete

[fricklerhandwerk]

This implements NixOS/rfcs#136. This issue is agreed-upon by the @nixOS/nix-team

Required changes:

• provide better information about why something was not deleted
  • example: Cannot delete store path with no roots #6141
  • e.g. provide in the error message what to do to get more information or print it directly – which root prevented GC, which path is still alive and why?
• add this behavior: nix store delete ./result should delete the linked store path, the symlink, and the GC root
  • currently it can't delete the store path because it's resolved first, and recognised as linked to, leading to an inscrutable error message
• document that resolving the argument to a store path takes precedence
  • example: if you have a flake in a store path, the argument is taken as a store path, not as a flake.
  • installable resolution is a global issue though, and we will address this in a global review pass of the CLI