feat(css): Add gl-top-full mixin

mentioned in commit 7eeb3885

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!243 (merged)

mentioned in merge request gitlab-docs!4463 (merged)

mentioned in merge request gitlab!139114 (merged)

mentioned in merge request gitlab!139832 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3665 (merged)

mentioned in merge request status-page!891 (merged)

mentioned in merge request status-page!893 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!21 (merged)