[NEW, CHANGE] Vertical alignment of "slide" content

[Rob Beezer]

Prompted by a request on pretext-support, you can now vertically align the
content of slides in a slideshow. top, middle, bottom. Thanks to Charles M for
putting this one forward.

"top" is the default. Because (a) this is the default for Reveal, (b) the other
options for Reveal require fancy CSS to effect; (c) Reveal has been functional a
lot longer than Beamer has. So a Beamer slideshow will now change from Beamer's
default "middle" look to its "top" look, that is the one and only change here.
Fix is trivial: one line in your #docinfo can turn that back to previous behavior.

docinfo/defaults/slides/@valign - document-wide setting

slide/@valign - per-slide override

Use case was room at the bottom to scribble while presenting in class.
Excellent motivation. Not sure "bottom" is useful, but included for symmetery,
and maybe you just want some caption-like sentence below a whole lot of
scribbling? Who knows exactly what goes on in the classroom?

Should some of this publisher file options? I had that argument with myself.
Then, I came to a decision, which I straight-up asked my assistant to implement.
It (amazingly?) raised the same discussion and came down (barely) on the same
side I did. Only an author knows which slide should have lots of blank space,
and which is some big central theorem that should be centered vertically. And
having a publisher switch the global default might be a disaster. Or, this is
akin to @workspace - an author decision.

See built examples at the website, look for section "Vertical Alignment" at the end.

Section 4.43: Slides (last paragraph)
https://pretextbook.org/doc/guide/html/topic-slides.html#topic-slides

Maybe even a little bit more at:
https://github.com/PreTeXtBook/pretext/pull/3137