Supersede `tidyr::separate` with `tidy::separate_wider_delim` once the latter is stable

[jdhoffa]

nit: tidyr::separate() is superseded by tidyr::separate_wider_delim()

Originally posted by @jacobvjk in #464 (review)

AB#9901

[jdhoffa]

Given that tidyr::separate_wider_delim is still an "experimental" function, probably not a good idea to implement this right away.

Note: to look back into this once the function is stable

[cjyetman]

noting that as of tidyr 1.3.1 (CRAN release: 2024-01-24), which is still the most recent version of {tidyr} on CRAN, tidyr::separate_wider_delim() is still considered experimental