[UPDATED] Offline MathJax to version 4

[Rob Beezer]

Straight HTML/online output has migrated recently to MathJax 4. We have now
upgraded our use of "offline MathJax" to version 4 as well. Many thanks to
Davide Cervone, MathJax founder and lead developer, for his assistance with this
transition.

(Notice how, as an author or a publisher, you are absolved from keeping up with
upgrades like this one. This is one of the original motivations for PreTeXt.)

We use this for math in EPUB, Nemeth braille in braille output, and just
recently for math in the "LaTeX No LaTeX" project making accessible PDFs. We
also embed "spoken math" into the EPUB and the accessible PDFs.

If you use the CLI, the upgrade should come to you in good time. If you are a
developer using the pretext/pretext script you will need to do the following:
from the script/mjsre directory, run ./update-sre. (It simply removes the
node_modules directory and runs a fresh npm install.)

What you might notice:

* Spoken math is refined in places: "normal infinity" becomes simply
"infinity", "c i s" is now the word "cis", and redundant layout announcements
around single-line displayed equations are gone.

* Nemeth braille follows the Speech Rule Engine's revised rule sets; in our
test corpus roughly 6 to 8 percent of expressions re-encode, generally toward
more uniform treatment (for example, coordinate pairs like (-1, 2) now encode
consistently whether or not they contain a minus sign).

* Math images (SVG) are drawn with MathJax's new-generation Computer Modern
font. Side-by-side renderings are visually indistinguishable; dimensions can
shift by hairline amounts.