Support deeper sub*sections for LaTeX

[fare]

subsubsub*section becomes bold in HTML, and I think in scribble/acmart too (though that's actually ugly in acmart). But in scribble/book it just drops the content. The underlying LaTeX macro in book/style.tex expands to {{\bf #2}} so I suspect what we produce expands to a single argument to the macro, but I admit I don't understand the scribble data model and its backends much.

[fare]

Googling around, I found this thing. I don't know what appetite there is to adopt that for Scribble, but personally I would love it: https://tex.stackexchange.com/questions/60209/how-to-add-an-extra-level-of-sections-with-headings-below-subsubsection

[mflatt]

I pushed a repair for the immediate problem.

For deeper subsections, it looks like the scribble Latex layer is prepared to support up to 4 "sub"s of depth, so a part-start would be created that has depth up to 4. It looks like the right thing would happen for a depth up to 3, and then a style file would need a \renewcommand{\Ssubsubsubsubsection}[2]{....} to configure depth 4. Beyond that, Scribble's Latex renderer would need to change.