Add pa11y-ci for automated accessibility tests

This is mostly a POC and it will need a lot more tweaking and some of my other PRs finished first, like #29208.

• Look into ignoring some classes known to fail in hideElements (*-success, etc) ("hideElements": "[class*=-success], [class*=-info]")
• Maybe ignore examples from color contrast check?
• See if we can use html reporter (https://github.com/pa11y/pa11y-reporter-html)
• Find a way to use the HTML reporter but still log the results in CLI
• See if we can drop delay-cli
• Upload HTML report on CI
• See if we can use npx maybe, since Chromium is a huge download or see if we can cache puppeteer on CI