Consider failing SEO category when page is blocked from indexing

[patrickhulce]

Summary
SEO category bears many similarities to PWA category in that a numeric weighted average score does not accurately reflect the result. e.g. if you've done all the optimization in the world but your robots.txt accidentally blocks the page from being discovered, you really should have a score of 0.

Ideas:

• Just increase the weight of is-crawlable to something very high (dilutes other results though)
• Bespoke logic to give category 0 if is-crawlable fails.
• Something fancier ala old PWA category passing criteria.

Context: https://twitter.com/filrakowski/status/1630246287202484224

[connorjclark]

Perhaps a necessary: boolean on AuditRef.

[paulirish]

after discussion: We'll increase the weighting of is-crawlable to ensure it's worth at least 31% of the seo score.