Provide output_folding YAML Setting

[DarioS]

Similar to the currently possible code_folding: hide, it would be convenient to have output_folding: hide be a possible setting. The use case is the creation of a tutorial sheet, but not using learnr because the students should familiarise themselves with RStudio code editing and R console windows and avoid the hassle of learnr deployment.

[cderv]

I believe this is related to https://github.com/rstudio/rmarkdown/issues/1453

Some formats based on rmarkdown already contains possibility of folding output. See for example the minidown package.

regarding teaching, learnr can be run locally I believe, and there are also alternative like unilur