feat(Typography): Add heading scale classes

mentioned in commit de52ece5

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!213 (merged)

mentioned in merge request gitlab!132987 (merged)

mentioned in merge request status-page!878 (merged)

mentioned in merge request gitlab-docs!4293 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3561 (merged)