Add footnote support

[nvanexan]

Hi maintainers 👋

I had need for support for footnotes in my own project. Submitting this as a draft to add support for parsing footnotes via the markdown-it-footnote plugin.

I added a test to the parsers but feel free to play with it a bit. If you think this could be useful for all, feel free to comment, edit or add as you see fit. Thx!

Example text

 foo[^1]

[^1]: bar

Example output

    <article>
      <p>
        foo<sup class="footnote-ref"><a id="fnref1" href="#fn1">1</a></sup>
      </p>
      <section class="footnotes">
        <ol class="footnotes-list">
          <li class="footnote-item" id="fn1">
            <p>bar<a class="footnote-anchor" href="#fnref1">↩</a></p>
          </li>
        </ol>
      </section>
    </article>

[CLAassistant]

All committers have signed the CLA.

[nvanexan]

Updated to remove the commits in a previous PR and to get the build check to pass. Cheers! 🍻

[rpaul-stripe]

I appreciate that you took the time to put this PR together, but I think we're going to pass on this one. We'd prefer not to introduce additional syntax or dependencies on additional markdown-it plugins, especially in cases where it would be practical to implement the feature with Markdoc tags.

[matildepark]

especially in cases where it would be practical to implement the feature with Markdoc tags

So are we to advise writers to use {% footnote %} format instead of just [^1]?

[mfix-stripe]

especially in cases where it would be practical to implement the feature with Markdoc tags

So are we to advise writers to use {% footnote %} format instead of just [^1]?

For now, yes. We will keep an eye on requests like these and see if they prevail. We prefer that users re-mix existing APIs than continually extending the syntax of Markdoc itself.

[matildepark]

For future readers, I ended up porting the work in nvanexan/nve2022#7 for my situation (double kudos to OP here).