- Add REGEXP_LIKE function that returns a boolean, and is useful in
  WHERE clauses.
- Fix REGEXP_EXTRACT return type (should be nullable; causes incorrect
  filter elision).
- Fix REGEXP_EXTRACT behavior for empty patterns: should always match
  (previously, they threw errors).
- Improve error behavior when REGEXP_EXTRACT and REGEXP_LIKE are passed
  non-literal patterns.
- Improve documentation of REGEXP_EXTRACT.