gh-92032: Add soft keywords to rlcompleter (#92029)

Let the interpreter autocomplete soft-keywords, ATM the PEP-634 'match'
/ 'case' / '_' (wildcard pattern).

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>