feat(CSS): include design token CSS custom properties in gitlab_ui.scss

mentioned in commit 9fcc9cfa

mentioned in merge request status-page!903

mentioned in merge request gitlab!156944 (closed)

mentioned in merge request gitlab!156947 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!601 (closed)

mentioned in merge request gitlab!157024 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!603 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!604 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3905 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!338 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!40 (merged)