docs(css): clarify regex negative lookahead analogy for :has() regarding last-child - #45087

Summary

This PR addresses and resolves #45038 by clarifying the analogy between regular expression negative lookaheads (?!pattern)\ and CSS :has()\ selectors.

Changes Made

• In \iles/en-us/web/css/reference/selectors/_colon_has/index.md, updated the Negative lookahead (?!pattern) section to explain that while .abc:has(+ :not(.xyz))\ handles the case where a next sibling is present and is not .xyz, it does not match when .abc\ is the last element (with no following sibling).
• Added the recommended CSS pattern (.abc:has(+ :not(.xyz)), .abc:last-child) to fully replicate the negative lookahead behavior for all cases.

Fixes #45038