Adding purple color variables to mirror GitLab UI

What does this MR do?

The $purple- color variables hadn’t existed in the product before, this adds them to mirror what’s in GitLab UI.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [- Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Relates to: gitlab-org/gitlab-services/design.gitlab.com!2076 (merged), gitlab-ui!1706 (merged)