Endow `str_flatten()` with the power to handle Oxford commas

[hadley]

Not sure what to do, but currently

stringr::str_flatten(letters[1:2], ", ", ", and ")
#> [1] "a, and b"

^{Created on 2022-06-27 by the reprex package (v2.0.1)}

[hadley]

Maybe just str_oxford()?