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 #11640.

Closes #11640
Closes denoland/divybot#557

Co-authored-by: divybot <divybot@users.noreply.github.com>
Co-authored-by: Divy Srivastava <me@littledivy.com>

Author: divybot