[yarn/npm] make package.json fig config more capable

[lukemelia]

Sanity checks

• My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue

What CLI tool does this relate to?

yarn/npm

Which statement makes the most sense?

This completion spec isn't complete

Issue Details

It would be helpful if the yarn/npm embedded fig config feature for package.json scripts had the ability to nest commands and add options. Currently, it only seems to support customizing the top-level suggestions.

[brendanfalk]

Totally agree - this should use the generateSpec prop in the run subcommand object as opposed our current implementation of having an argument beneath run!