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.
Certain lifted reference forms carried the prefix of the original
section, but in general the section needs to be similarly carried for
rendering elements that may contain references.
Also, fix the contract on `elemref` and `elemtag` to use
`taglet?` instead of `tag?`.
Thanks to Dupéron Georges.
