[NEW] Upgrades to Reveal.js slideshows

[Rob Beezer]

A big push today on Reveal.JS (HTML) slideshows.

An upgrade first, as suggested by Mitch Keller. Reveal.JS is now at v6. That
project began supporting MathJax 4 about the same time we did, and so we have
caught up there also. Thanks, Mitch, for the nudge.

The setup for making EPUB, braille, and LaTeX No LaTeX uses "offline" MathJax.
We have leveraged that to make the math in a slideshow as SVG images, so your
#slideshow will not need to have an internet connection to get MathJax, and may
also serve as a better archival version. We have added speech strings for
accessibility, but this may be substandard compared with MathJax's "explorer"
feature.

Then, channeling, Oscar Levin, I thought, "Why not fold in the Reveal.js JS and
CSS into the one HTML file?" So that is an option and your slideshow can run in
a browser as a single file without network access. Some cavets - image files
are separate, syntax highlighting needs online access, Sage Cell needs the
network, etc.

Plus, a little bit of work to make Reveal.js's "print to PDF" feature work
better for us. Still a small hack (on teh Reveal.js side). Read the
documentation for more on that.

Rob

Section 34.1: Reveal.js
https://pretextbook.org/doc/guide/html/publisher-revealjs.html#publisher-revealjs

https://github.com/PreTeXtBook/pretext/pull/3058

https://github.com/PreTeXtBook/pretext/pull/3060

https://github.com/PreTeXtBook/pretext/pull/3061