How to use `pagedown::chrome_print()` in a Shiny app?

This is a demo project to show how to use pagedown::chrome_print() in a Shiny application.

Using pagedown::chrome_print() in Shiny is not straightforward because:

async = TRUE must be used
in that case, pagedown::chrome_print() returns a promise
using promises in Shiny is an advanced topic for many users
please, read https://rstudio.github.io/promises/articles/shiny.html

This demo app is the first step for writing a vignette on this topic (see rstudio/pagedown#114).