fix(test): handle escaped markdown code fences (#35125)
Fix markdown doctest extraction so fenced blocks are matched using the
opening backtick fence length. This prevents literal triple-backtick
snippets inside longer escaped fences from being extracted as doctests.
Adds a focused `deno test --doc` regression for the escaped-fence
markdown from denoland/deno#11640.
Closes #11640
Closes denoland/divybot#557
Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>
