Text Fragments: Scroll the start container of a range into view instead of its common ancestor.
Spec issue: WICG/scroll-to-text-fragment#259
Differential Revision: https://phabricator.services.mozilla.com/D216147
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1906895
gecko-commit: 91c93776446c64bb2438b0fa69d0b9f198bb00a9
gecko-reviewers: emilio