Make `add_tally()` S3 generic?

[etiennebacher]

count(), add_count(), and tally() are generics but add_tally() is not:

sloop::is_s3_generic("count", env = getNamespace("dplyr"))
#> [1] TRUE
sloop::is_s3_generic("add_count", env = getNamespace("dplyr"))
#> [1] TRUE
sloop::is_s3_generic("tally", env = getNamespace("dplyr"))
#> [1] TRUE
sloop::is_s3_generic("add_tally", env = getNamespace("dplyr"))
#> [1] FALSE

Is it possible to make add_tally() generic?

It looks like this was already planned in #3739 but add_tally() was forgotten.