Add support for external formulae

[SteveLLamb]

Specify using <math> for smaller snippets inline, as per already supported https://doc.smpte-doc.org/html-pub/main/#sec-formulae, and add support and instructions for tex as external snippet for longer formulae, using editors judgement.

[SteveLLamb]

@ccameron-chromium, can you provide what you need exactly to fulfill the work-around we discussed here ccameron-chromium@7822825?

[ccameron-chromium]

I'm not sure what the right solution is here.

I had previously suggested something to the effect of <math data-include="..."></math>, but I've since realized that that's too cumbersome.

The issue is that div of class formula is required to have a single <math> child, but I'm not able to get MathJax to satisfy that. When I write the source, I write something like:

  <div class="formula" id="eq-1">
  \[a=b+c\]
  </div>

This has no children (just inline text), and MathJax comes along and asynchronously replaces the \[a=b+c\] with a <math> element (inside a mjx-container element, to add to the fun).

The best fix may be to delay the SMPTE validation code from running until MathJax has done its various "work" on the DOM. I'll see if I can manage to make that happen.

[palemieux]

See also #156

The basic question is do we want to support TeX formula input when MathML exists? This would create a dependency on MathJAX since TeX input not supported natively by browsers.

[ccameron-chromium]

I had assumed that MathJax was sanctioned, based on its inclusion in the instructions ("The head element may contain additional script elements that import JavaScript libraries for document layout, e.g. MathJax").

I had originally tried to use MathML, but it's too verbose to be used directly for nontrivial expressions.

I also found that MathJax' rendering of equations was much better than raw MathML (unfortunately there are still lots of MathML bugs across basically all browsers except for Firefox).