Add `.keep` to `mutate()`

[markfairbanks]

library(dplyr, warn.conflicts = FALSE)

df <- tibble(x = 1:3, y = 1:3)

df %>%
  mutate(double_x = x * 2, .keep = "used")
#> # A tibble: 3 × 2
#>       x double_x
#>   <int>    <dbl>
#> 1     1        2
#> 2     2        4
#> 3     3        6