Add support for task lists (checkboxes) and footnotes in MarkdownViewer

[xarlstrat-cyber]

Hi!

The built-in MarkdownViewer plugin currently doesn't render two common GitHub Flavored Markdown (GFM) features:

1. Task lists / checkboxes
   Input:

• Task not done
• Task done
  Expected: rendered as actual checkboxes (☐ / ☑), like on GitHub.
  Current behavior: rendered as plain text with a bullet point, e.g. • [ ] Task not done.

2. Footnotes
   Input:
   Some text with a footnote¹.

QuickLook version: 4.5.0 (latest as of testing)
OS: Windows

Both features are part of standard GFM and are commonly used in README files and technical documentation, so supporting them would make MarkdownViewer's output more consistent with how .md files render on GitHub.

Footnotes

1. This is the footnote explanation.
   Expected: ¹ rendered as a small superscript reference link, with the definition moved to the bottom of the document as a footnotes section.
   Current behavior: ¹ and the ¹: ... definition line are both left as literal plain text in place. Additionally, footnote references seem to get picked up by the document outline/TOC generator, producing garbled entries in the sidebar (e.g. random numeric IDs instead of headings).
   Environment ↩ ↩² ↩³ ↩⁴