nushell: Add a janky `command_not_found` hook.

Allows falling back to a package from nixpkgs if the entered command doesn't exist.

This works OK but as far as I can tell, it's impossible to stop Nushell from outputting the underlying error message after the hook runs…

Created by  PlumJam  on November 17, 2025
LSXN66FT2WHIINRJM4DM2DBJVFZ2XK77D2JRC665ECG4N32ITTXQC
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