Add package.json support

[derekgreer]

I'd like to be able to do this in a package.json script:

command-exists XYZ || 'echo installing command XYZ'

Can this be done?

[mathisonian]

@derekgreer, it could be done by creating a smaller wrapper around this module. In particular, node packages can expose command line commands like that (see, e.g. https://developer.okta.com/blog/2019/06/18/command-line-app-with-nodejs).

I don't have time to implement at the moment, however would be happy to review a PR.