Exclude rel=nofollow links from prefetch/prerender

[westonruter]

Summary

WooCommerce has rel=nofollow on its Add to Cart buttons (which are often links instead of button elements). These links are not idempotent and they must be excluded from prefech/prerender.

Fixes #1140

Before	After

Relevant technical choices

Alternatively this could have excluded URLs that include the add-to-cart query parameter. However, this would seem to be overly-specific to WooCommerce. The issue WICG/nav-speculation#309 was opened by @tunetheweb to consider whether rel=nofollow links should be excluded by default as well, so that is what this PR is going with now as well.

Eventually there may need to be a filter in addition to plsr_speculation_rules_href_exclude_paths which applies on the entire speculation rules array so that a plugin can add exclusions which are not URL-specific.

Before

{
  "prerender": [
    {
      "source": "document",
      "where": {
        "and": [
          {
            "href_matches": "/*"
          },
          {
            "not": {
              "href_matches": [
                "/wp-login.php",
                "/wp-admin/*"
              ]
            }
          },
          {
            "not": {
              "selector_matches": ".no-prerender"
            }
          }
        ]
      },
      "eagerness": "moderate"
    }
  ]
}

After

{
  "prerender": [
    {
      "source": "document",
      "where": {
        "and": [
          {
            "href_matches": "/*"
          },
          {
            "not": {
              "href_matches": [
                "/wp-login.php",
                "/wp-admin/*"
              ]
            }
          },
          {
            "not": {
              "selector_matches": "a[rel=nofollow]"
            }
          },
          {
            "not": {
              "selector_matches": ".no-prerender"
            }
          }
        ]
      },
      "eagerness": "moderate"
    }
  ]
}

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: adamsilverstein <adamsilverstein@git.wordpress.org>
Co-authored-by: tunetheweb <tunetheweb@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[adamsilverstein]

Looks good, one small doc typo/suggestion. Also, the tests are complaining about your changes.

[tunetheweb]

LGTM with one suggestion.