add support for the regex `d` flag

[CogentRedTester]

Add support for the regex d flag

• [ ] This PR only introduces changes to documentation.
  • Please include a summary of changes and an explanation
• [x] This PR adds new functionality
  • [ ] Is there a related issue?
    • Please note the related issue below, and how this PR relates to the issue if appropriate
  • [x] Does the code style reasonably match the existing code?
  • [x] Are the changes tested (using the existing format, as far as is possible?)
  • [x] Are the changes documented in the readme with a suitable example?
  • [x] Is the table of contents updated?
  • [x] Is the index.d.ts file updated, using appropriate types and using the same description as the readme?
• [ ] This PR introduces some other kind of change
  • Please explain the change below

A simple method that just adds support for the one flag which remains unsupported.