test(docs): improve style guide URL path test

[shawnbot]

This refactors our docs site test command to more intelligently list the Primer-specific paths from the Style Guide site with the ones in our pages/css directory.

In a nutshell: Rather than hacking together a path list with the file command, there is a Node script that looks at a list of paths committed to before.txt (or, if running locally, it looks for a github/styleguide checkout, builds the site, and generates a new before.txt from the list of built paths — and compares it to the list of "pages" in pages/css. The test script fails if there are paths in the former that don't exist in the latter, and which aren't accounted for by any "exceptions" (see below).

Exceptions

Because we're changing some URLs, there are bound to be paths that don't match up. I thought it'd be good to explicitly list these in a way that's easy to understand, is testable, and helps us add the correct redirects from the styleguide when we're ready to decommission it. The exceptions object in docs/test/urls.js does this, e.g. to account for the status key move in #635:

const exceptions = {
  '/whats_new/status-key': moved('/status-key')
}

This could be read as:

The Primer style guide path /whats_new/status-key (/primer/whats_new/status-key) has moved to to /status-key in the Primer CSS docs site (/css/status-key).

Once this PR passes CI, I think it's fair to say that we've accounted for most of the Primer pages in the style guide and "documented" any that aren't going to exist in the new site, checking off a bunch of boxes in #628. 🎉

[jonrohan]

Looks ok to me