feat(Type): add line-height and letter-spacing mixins for headings

mentioned in commit 35418801

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!154 (merged)

mentioned in merge request status-page!811 (merged)

mentioned in merge request gitlab!123721 (merged)

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

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

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

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