feat(css): Add easing variables

mentioned in commit 55047c39

mentioned in merge request status-page!903

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!26 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!300 (merged)

mentioned in merge request gitlab!147114 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3771 (merged)