feat(DesignTokens): add dropdown design tokens

mentioned in commit e3510dda

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!348 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1086 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!167862 (closed)

mentioned in merge request gitlab!168179 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4098 (merged)

mentioned in merge request gitlab!168368 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!54 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)