Feature Proposal: count number of issues by issue code

[sgregson]

❤️ the work on pa11y@5 beta!

I've been playing around with the beta and found myself often working to facet the issues list by their code. Other a11y testing tools provide this view to help prioritize fixes - the more issues of a type, the more critical to address first.

I worked on a local branch of the reporter to produce the following mock of what I had in mind, aiming for:

• Facet issue counts (split by error,warning,notice) according to Success Criteria, sorted by volume of issues
• no clientside scripting
  • uses the HTML <details> element (IE/Edge support TBD) to reveal/hide
• link directly to WCAG2 Success Criteria
  • via regex extracting from issue.code.split('.').filter(chunk => chunk.match(/^[A-Z]+\d+$/)), which is gross-but-works. would love better method.

I'd be happy to add some tests and bring it closer to PR-ready if that's in keeping with the goal of this reporter?