Add highlight design tokens

This change adds new design tokens for highlighting elements in the GitLab UI. It introduces two types of highlights: "match" highlighting (using orange colors) for search results and autocomplete suggestions, and "target" highlighting (using blue colors) for referenced elements like URL anchors. Each highlight type includes background and text/border color variables that work in both light and dark themes. These tokens will help maintain consistent visual styling for temporary emphasis across the application without implying any particular state or meaning to users.

Related to !4478 (merged) !4452 (merged) #1990 (closed) #2008 (closed)