MathJax 4 upgrade

[Rob Beezer]

There is now a PR with a serious attempt at the upgrade to MathJax 4. (For
in-browser rendering only, offline mode for EPUB and braille is still waiting on
a stable release).

https://github.com/PreTeXtBook/pretext/pull/2818

I would really like to identify any blockers ASAP, meaning in the next 10 days
or so. I'm trying to coordinate with Oscar on CLI releases, with the idea that
this could go into general circulation in early May.

And if you have ideas about new features we could support, or minor annoyances
that can now be addressed, it should be safe to build branches off this for
eventual PRs.

Technical comments can go on #2818, higher-level discussion can go here. Thanks
for your assistance with a major upgrade to a key piece of the puzzle.

Rob

[Rob Beezer]

Forgot to mention. With MJ4, when you click on some math, you are immediately
thrown into the expression Explorer (which is a great accessibility feature).

However, you also click on display math to open a knowl embedded within (one of
my favorite features). And in "presentation" mode, a click will zoom the math
to be large on the screen. Those features still behave, but you get a highlight
on a portion of the math as parto of exploring, which seems to persist (and you
get a little info icon about how the Explorer works).

I don't think this is a blocker (so I have not investigated our options), but I
suspect it is something we want to improve quite quickly.

If you are testing, first, thank-you very much, and second, there are now some
updates available from Davide Cervone (thanks, Davide!).

Rob