Wrapper around getOption("readr.show_col_types") that implements some fall
back logic if the option is unset. This returns:
TRUEif the option is set toTRUEFALSEif the option is set toFALSEFALSEif the option is unset and we appear to be running testsNULLotherwise, in which case the caller determines whether to show column types based on context, e.g. whethershow_col_typesor actualcol_typeswere explicitly specified
