[CHANGE] Inferior slanted fractions, the \sfrac math macro

[Rob Beezer]

Only of interest if your source uses the "\sfrac" macro for slanted, or beveled,
fractions. And only a change for LaTeX output.

The \sfrac{}{} macro has seen varying levels of support since we initiated
internal automatic support in 2015. The past several years the result in
MathJax-based outputs has just been the numerator and denominator separated by a
slash. Nothing fancy.

As of just now, the same is true for LaTeX, nothing fancy. But wait! It is now
simply the author's responsibility to load the relevant package for superior
results, instead of PreTeXt doing it stealthily for you. We are getting out of
that business. And it took me hours to figure out how to do this without a
breaking change. ;-)

Add the following to your "docinfo" as announced a few hours ago, and as is now
done in the sample article.

<math-package latex-name="xfrac" mathjax-name=""/>

There are hints MathJax might be doing something similar and we'll provide an
upgrade path for that then too.

See:

Subsection 4.9.18: Extras
https://pretextbook.org/doc/guide/html/topic-mathematics.html#subsection-51

Rob