Add support for "labeled" footnotes

[jeffkreeftmeijer]

Newer versions of Org¹ uses footnote labels as their anchors.

Hello, world![fn:labeled]

[fn:labeled] A footnote.

In previous versions, the example above would produce a footnote with fn.1 as the anchor:

<ol>
  <li id=\"fn.1\" class=\"footdef\" role=\"doc-footnote\" tabindex=\"-1\">
    <p class=\"footpara\">A footnote.</p>
    <p class=\"footpara\">\nWith a second paragraph.\n</p>
    <a href=\"#fnr.1\" role=\"doc-backlink\">↩&#65038;</a>
  </li>
</ol>

This produced broken links, as Org now uses fn.labeled when linking to this footnote from the footnote reference.

This patch uses the label if available and falls back to numbered foonotes. The example above now produces a "labeled" footnote:

<ol>
  <li id=\"fn.labeled\" class=\"footdef\" role=\"doc-footnote\" tabindex=\"-1\">
    <p class=\"footpara\">A footnote.</p>
    <p class=\"footpara\">\nWith a second paragraph.\n</p>
    <a href=\"#fnr.labeled\" role=\"doc-backlink\">↩&#65038;</a>
  </li>
</ol>

¹ ox-html-markdown-style-footnotes now targets Emacs 30.2, which bundles with Org 9.7.13.