Summarize w/ across(): setup

df <- tibble(a = rnorm(10), b = rnorm(10), c = rnorm(10))
glimpse(df)

## Rows: 10
## Columns: 3
## $ a <dbl> -0.17905159, 1.90236182, -0.10097489, -1.35984070, -0.66476944, 0.48…
## $ b <dbl> 1.59850877, -0.08856511, 1.08079950, 0.63075412, -0.11363990, -1.532…
## $ c <dbl> 2.29307897, 1.54758106, -0.13315096, -1.75652740, -0.38877986, 0.089…