Integrate clap_complete_nushell

[casey]

The current nushell completion script, in src/completions.rs NUSHELL_COMPLETION_SCRIPT only completes recipe names. We should integrate clap_complete_nushell, so we can also complete flags and other arguments. Note that clap_complete_nushell doesn't complete recipe names, so the output script will have to be modified in order to also complete recipe names.