Add `revert-rule` keyword

It is being added to Chromium ([intent to ship](groups.google.com/a/chromium.org/d/msgid/blink-dev/CAKFBnUpcohOCEHaFeoDUh%2B49qF-UfRzHT-eQopUzBf8Myo%3Dfog%40mail.gmail.com?utm_medium=email&utm_source=footer)). **Explainer** https://github.com/w3c/csswg-drafts/blob/main/css-cascade-5/revert-rule-explainer.md **Specification** https://drafts.csswg.org/css-cascade-5/#revert-rule-keyword **Initial public proposal** https://github.com/w3c/csswg-drafts/issues/10443 **TAG review** https://github.com/w3ctag/design-reviews/issues/1193