Add optional math support for markdown

[recmo]

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This fixes #2019 without adding substantial complexity or dependencies. It optionally enables a common-mark extension available in pulldown-cmark.

This will correctly parse delimited math, and render html like <span class="math math-inline">\sum_i x_i<\span>.

To render the latex using Katex, include something like this in the header:

    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.11/dist/katex.min.css"
        integrity="sha384-nB0miv6/jRmo5UMMR1wu3Gz6NLsoTkbqJghGIsx//Rlm+ZU03BU6SQNC66uf4l5+"
        crossorigin="anonymous">
    <script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.11/dist/katex.min.js"
        integrity="sha384-7zkQWkzuo3B5mTepMUcHkMB5jZaolc2xDwL6VFqjFALcbeS9Ggm/Yr2r3Dy4lfFg"
        crossorigin="anonymous"></script>
    <script defer>
        document.addEventListener("DOMContentLoaded", function () {
            const macros = {};
            for (let el of document.getElementsByClassName('math')) {
              katex.render(el.textContent, el, {
                output: 'html',
                throwOnError: false,
                displayMode: el.classList.contains('math-display'),
                macros,
              });
            }
        });
    </script>

[Keats]

We want that to be an enum. Eventually (hopefully) there will be math rendering inside Zola (eg https://zola.discourse.group/t/a-simpler-katex-alternative-that-works-today/2156/3) so we will want to support classes/pre-render etc