Restore internal/x/expander

Keep the star expander package around for future use. It is
self-contained: its ColumnGetter interface is defined locally and its
tests bring their own implementations, so it does not depend on the
removed analyzer methods.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JyDtVLiKKixbPi6YvdM5wd

Author: claude