Add a `link-render-style` syntax property to control the rendering of
section links --- so that HTML output can say "section <number>", and
so that Latex/PDF output can have just the section number hperlinked
(as in acmart).
It seems unfortunate that the link rendering is so hardwired into each
rendering back-end, but maybe this can be made even more configurable
in the future. Meanwhile, Latex macros already provide an additional
layer of rendering control (but not enough, it turns out, to easily
perform the same adjustments as the 'number mode that matches acmart).
For `scriblib/figure` make `figure-ref` and `Figure-ref` similarly
sensitive to the link-rendering style.
For `scriblib/autobib`, change the hyperlinking of references
so that the color can be overridden, and make `scribble/acmart`
override it.
If a value is convertible to 'text, then use that conversion.
Otherwise, convert using `write` instead of always using "???".
Also, correct documentation to include convertible values among
the valid forms of content, and document the new conversion rules
there.
I'm not certain that mixing 'grouper and non-'grouper parts at a given
level makes sense. Still, this adjustment brings Scribble section
counting more in line with Latex, so that section links are less
likely to have the wrong number label.
* Add a pretitle style property for nested flows.
This allows us to raise nested flows above the title. So that we do
things like raise the abstract above the title:
```
\begin{abstract}
Abstract text
\end{abstract}
\titleCommand{...}
```
This style is required by the acmart style guide.
* Paragraphs and nested flows extracted in the same order
* Fix `scribble/acmart`'s abstract form so that it gets lifted above `maketitle`, where it should be.
