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.
