Convert #slideshow to Beamer

[Rob Beezer]

The US government allowed Anthropic to resume making the Fable 5 model available
a few days ago. So I did what any rational PreteXt developer would do - I
switched to that model and asked it to run non-stop and make a new PreTeXt
conversion. Of course, why not?

Well not entirely new. Revamp the existing conversion of a #slideshow to a
Beamer PDF. I said that nothing about the existing conversion was sacrosanct.
A few hours later, with no additional input from me, and I was getting a pretty
good looking Beamer PDF out, and little of the original was left.

I had been content to let this conversion wither away, due to a lack of
interest, or worse, neglect. But outputting to Beamer does have some advantages
compared to RevealJS, such as (a) no need for an internet connection to get
MathJax loaded, and (b) a faithful immutable PDF as an archival copy (rather
than an HTML file with a lot of CSS and JS baggage).

So we will try a new development model on this one. If you see a part of the
conversion that needs improvement (like the excess line-numbers I see in the
#program #listing), or you think there needs to be a new feature (eg, Beamer
themes in the publication file!), do the following. Start a discussion on
pretext-dev if you think it is a decision that needs discussion, and then make
an issue on GitHub. But make the issue very, very specific, like you are
talking to a 5-year-old, so that an AI agent can read it and make the changes
properly. I will supervise that process and review the changes before merging.

Available now via the pretext/pretext script. And not documented anywhere
else. Yet.

@Oscar - there has been a signature change that should affect formats that
produce a PDF via a (la)tex executable. Should be apparent in the commits,
otherwise ask for more info.

Rob