Update the standalone scripts

[ddsjoberg]

Call usethis::use_standalone() to update the standalone scripts that previously lived in my personal GH, to the versions in https://github.com/insightsengineering/standalone .

NOTE: The file with the functions that check package installations have an updated API. The default reference_pkg = NULL argument value has been updated to ref = packageName(). The arg name needs to be updated throughout the package. Where we see reference_pkg = "gtsummary", we can fully remove the argument (since the default is what we need).