feat(Typography): Add font-size variable map and heading mixins

mentioned in commit 453c8e87

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!210 (merged)

mentioned in merge request status-page!875 (merged)

mentioned in merge request gitlab-docs!4273 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3557 (merged)

mentioned in merge request gitlab!132987 (merged)