Parse and serialize `view-transition-class: none | <custom-ident>+`. This property will be used after we extend `<pt-name-selector>` to `<pt-name-and-class-selector>` for named view transition pseudo-elements. Will do this in Bug 1956646. Also, we will handle the class matching in Bug 1964949. Differential Revision: https://phabricator.services.mozilla.com/D249681