Force `.f` argument usage in epix_slide

[dshemetov]

• issue?: unclear error message here

archive_cases_dv_subset %>% group_by(geo_value) %>% epix_slide(.before = 6, tibble(slide_value = mean(.x$percent_cli)))
Error: object '.x' not found

We should probably capture + generate a better error message by forcing .f at the beginning; a fix is to insert an extra comma so this is sent to ... rather than .f and can be considered for tidyeval unnamed tibble column unpacking. Seems like probably an issue that already existed. It'd be nice to just let this expression through in .f, but I think I already have a dedicated separate issue for this and it is not as simple as it sounds.

Originally posted by @brookslogan in #477 (comment)