Add Claro administration theme to core

admin UI (the Seven theme) feels outdated and needs a visual update. The current designs were created for Drupal 7 in 2008-2009 and were slightly updated for backdrop CMS

Claro was validated in contrib and has thousands of sites using it, and several Drupal distributions have already adopted it. Claro has been implemented using the new Drupal Design System.

Claro is now part of Drupal core

https://github.com/backdrop/backdrop-issues/issues/4410