Support underscore in chunk label name with referencing mechanism

[cderv]

From https://github.com/ropensci/targets/discussions/469#discussioncomment-824093

Possibly the only place that we need to modify in bookdown is here if we want to support _:

https://github.com/rstudio/bookdown/blob/c8883c9bbd70400144b396b7cfc66e5e4f2ae8a5/R/latex.R#L117-L130

\@ref(foo_bar) would become @ref(foo\_bar) in .tex output. We need to consider this \ before _.

[cderv]

Related to

• #892,
• #839,
• #157,
• #941,
• #867,
• #406,
• #194
• #336

[cderv]

I've look into this more and it is not as straightforward as I thought. Underscore in LaTeX seems to cause issue, even when escaped by \_ and it is not just a matter of changing the regex.

I can't make it work in LaTeX right now. I'll look again later.