Add a packages command to get package info

[dkniffin]

While setting up my app, I've found a few times where I want to know what packages I have in my importmap, as well as their pinned version. I could use a regex to get that myself, but this feels like something that should really be built into importmap-rails.

[dkniffin]

I set up a repo explaining how I'm using this new command: https://github.com/dkniffin/importmap-rails-package-checker

I would love to see something like that functionality integrated into importmap-rails. In general, the dev tooling for importmap-rails is still lacking. It's unclear how to manage dependencies and subdependencies or how semver and upgrading is supposed to work or how JS unit tests should work, or security checking.

[dhh]

I'm not sure what this offers over just looking at the import map file itself?

[dkniffin]

@dhh It adds a programatic way to get the version numbers, for any number of purposes.

My main use-case is to keep importmap versions in sync with package.json, because I still need that to stay around for JS unit testing (since Node doesn't currently support importmap). But I'm sure there are many other reasons to need the current JS packages listed in the CLI.