Downloads R in app_dir. If it has already been downloaded, get_R will use that file. If the download fails it will stop.
get_R(app_dir, R_version)Shiny app's directory, defaults to getwd().
R version to use, defaults to: paste0(R.version$major, '.', R.version$minor).
sprintf('R-%s-win.exe', R_version) in app_dir.
If create_app(include_R = TRUE), then get_R.
get_R, copy_installation, create_config, create_bat, create_pkgs, directives, setup, languages, tasks, files, icons, run, and code.