Make a headless Chromium C++-backed package

[hrbrmstr]

Ref: https://chromium.googlesource.com/chromium/src/+/lkgr/headless/README.md

It'd likely be a massive memory hogging pkg but it ostensibly shld work.

Not sure how doable it is in the time frame, tho.

[jcheng5]

This could be a start: https://github.com/rstudio/webdriver

cc @wch @gaborcsardi