Skip eval=FALSE chunks

[MichaelChirico]

Resurrection of #581.

As noted there, we can't get a fully general solution, as run-time usage of opts_chunk$set(eval = FALSE) is very hard to detect.

Still, I think it's feasible for us to look at chunk-level options as an improvement over status quo. The key will be using knitr:::parse_params():

https://github.com/yihui/knitr/blob/e8aae19fcc7443b8a212c8dc65ba2b83894f1fe0/R/parser.R#L174

which takes as input a string like "x, eval = stop('haha')" and outputs a key-value list like list(label = "x", eval = quote(stop('haha'))).

Parsing chunk params is also something we want for #773.

At a glance, using eval = FALSE is a very common way to tell knitr to skip R chunks:

https://github.com/search?q=filename%3ARmd+%22eval+%3D+FALSE%22

It looks like GH search can't help us find "```r" usage; I also don't see any usage of opts_chunk$set(eval = FALSE):

https://github.com/search?q=filename%3ARmd+%22opts_chunk%24set%22%2C+%22eval+%3D+FALSE%22

So I think this issue is pretty high-impact & value, even if imperfect.

[MichaelChirico]

This is coming up a fair amount for library_call_linter(), where in addition to #2161's issue with setup chunks, there are often eval=FALSE chunks up front advising the user how to install the package. Unless we can skip those chunks, library_call_linter() will be of limited utility to Rmd files.