Convert user input into label formatting function
resolve_formatter(format_labels, env = caller_env(n = 2), call = caller_env())
a function
argument passed to make_labelled
the environment in which to look up functions if
format_labels
contains character elements. The default,
caller_env(n = 2)
, uses the environment from which the user called
make_labelled()
the calling environment to use in the error message