Broadens the built-in set with the string/presence checks that came up
most often when reaching for this package:
- Required[T comparable] non-zero value
- Prefix / Suffix / Contains strings.HasPrefix/HasSuffix/Contains
- Match(*regexp.Regexp) arbitrary pattern
All follow the existing shape: a constructor returning func(T) error, a
sentinel error matched with errors.Is, table-driven tests and an Example.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
