Alias .formula2varlist for old R

[MichaelChirico]

          I've tried \method{sort_by}{data.table}(x, y, \dots) together with

\alias{sort_by.data.table} and, surprisingly, it just worked on both
R-4.2.2 and R-3.3.0. There's no need for scary-looking kludges like
\if{\Sexpr[stage=install,results=rd]{as.character(exists("sort_by",
"package:base"))}{\method{...}}. R CMD check only verifies that all
exported symbols are documented, not that all documented aliases are
exported (or even exist in the namespace).

The old versions of R do complain about missing .formula2varlist.

Originally posted by @aitap in #6679 (comment)

[MichaelChirico]

Follow-up to #6679