[CHANGE] Extensible arrows via the "extpfeil" LaTeX package

[Rob Beezer]

We have long provided direct internal support for the math macros creating
extensible arrows via the "extpfeil" LaTeX package, as a result of one of the
very first feature requests. There is a reliable MathJax extension (good). But
the LaTeX package has some dubious practices as a package, and so has caused a
few headaches. It was last updated 14 years ago.

So it is now an author's responsibility to consciously elect this package, as
described in recent posts here. Or see:

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

But we are not throwing you under the bus. (Would we ever?) If you do nothing,
your project will build just like before (only a small relocation of package
loading on the LaTeX side). We have preserved testing in the sample article,
but via an author election. However, we will only try to support the new way,
and the old way may go away at the first sign of further trouble. Furthermore,
accurate testing of the necessity and use of the old way would be a real fiddle.
So in contrast to the usual procedures, you will not get any warnings now, and
this is the only notice.

That's it - we are (mostly) out of the business of providing direct internal
support for extra (unreliable) packages extending the range of possible LaTeX
macros available in mathematics.

[Rob Beezer]

Two and a half years ago we said: "the old way may go away at the first sign of
further trouble"

We are there now. This package prevents using an alternate Unicode mathematics
font for LaTeX/PDF output, since it requires the "amssymb" package and that
conflicts with the "unicode-math" package.

We also said: "So it is now an author's responsibility to consciously elect this
package"

That is now required. We have removed the backward-compatibility of
automatically loading the "extpfeil" package.

We have also removed testing from the sample article so that we can use this
document to test alternate Unicode mathematics fonts.

Please use "pretext-support" if you need help getting a project back on-track.

Rob