feat(DesignTokens): add button design tokens

mentioned in commit 3ce0c0e8

mentioned in merge request status-page!903

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

mentioned in merge request gitlab!161979 (closed)

mentioned in merge request gitlab!158430 (merged)

mentioned in merge request gitlab!162004 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!3996 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!811 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!49 (merged)