Add coverage for list columns in vis_miss() vis_dat() and vis_guess()

[cregouby]

and thus fix #136 fix #137 and fix #139

library(dplyr, warn.conflicts = FALSE)
data("starwars")
library(visdat)
vis_miss(starwars)

^{Created on 2021-09-11 by the reprex package (v2.0.1)}

library(visdat)
data("starwars",package = "dplyr")
vis_dat(starwars)

^{Created on 2021-09-11 by the reprex package (v2.0.1)}

library(visdat)
data("starwars",package = "dplyr")
vis_guess(starwars)

^{Created on 2021-09-12 by the reprex package (v2.0.1)}

[njtierney]

amazing, thank you!