feat(Typography): add text size scale variables

mentioned in commit 89749658

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!155 (merged)

mentioned in merge request status-page!815 (merged)

mentioned in merge request gitlab!123927 (merged)

mentioned in merge request gitlab-docs!3975 (merged)

mentioned in merge request gitlab-docs!3977 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3432 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3479 (merged)