Desktop: Fixes #13549: OneNote importer: Support converting checklists to Markdown

[personalizedrefrigerator]

Summary

Updates both the Markdown converter and the OneNote importer to allow imported checklists to be converted to Markdown.

Partially fixes #13549.

Testing

• This pull request includes an automated test.
• It has been manually verified that:
  1. The checkboxes_and_unicode.one file can be imported successfully
  2. The "Test Todo" note includes a checklist, before converting to HTML (with images for checkboxes).
  3. The "Test Todo" note can be converted to Markdown, with the HTML checklist converted to a Markdown checklist.