Braille conversion, progress.

[Rob Beezer]

(Cross-posting, -dev, and -a11y)

I've made some big changes in the code for the conversion to braille. All
under-the-hood, except that page size (for an "embossable" BRF) can now be
specified in a publication file:

<braille>
<page width="32" height="27"/>
</braille>

Otherwise this should help alleviate some known bugs, and open up some aspects
of this conversion that were proving difficult. There are some PreTeXt elements
that have gone un-implemented for a while (the difficult ones), resolving those
will be the next big push (and I'll report here).

https://github.com/PreTeXtBook/pretext/pull/3037

Rob

[Rob Beezer]

And now, the promised follow-up improvements. Principally lists are in much
better shape, nested lists had been a real challenge under the old architecture.
Thanks, Claude.

https://github.com/PreTeXtBook/pretext/pull/3042

Still some loose ends:

* No index, formatting may be a challenge.

* Bibliography may need attention.

* Perhaps a thorough review to double-check compliance with BANA standards.

But, all in all, I think we have a very servicable conversion going. There is
some parallel work to convert PDF and LateX to PreTeXt which could greatly
increase the amount of material available. To say nothing of Oscar's
announcement of progress with the pandoc writer.

Rob