new audit: only a single h1 on a page

**Feature request summary**

SEO/Best practice check: the appearance of H1s in a page.

Willing but not able I'm afraid. I'm a HTML/CSS guy with extremely limited knowledge of JS :)

**What is the motivation or use case for changing this?**
I'm using Lighthouse for quick-checking my code. It reported a 100% score for SEO but it turned out there was a misconfiguration in my components that caused the H1s to render as H2s. While actual SEO impact always involves some guess work, it's an age old best practice to have your H1s mark up the main heading of a page. It's fairly easy to check (is there one or is there none) I think.

Also, it might be useful to check if there's ONLY one, since I believe the system with multiple H1s has been made unofficial again, with search engines pledging they'll follow this decision in months to come.

**How is this beneficial to Lighthouse?**
It gives more trust in the SEO/best practice scores. I think this is a generally accepted implementation with generally accepted benefits, when Lighthouse doesn't report it, it kind of makes me question the other things it might be missing.