Better warn on unseed levels

[EmilHvitfeldt]

To close #450

library(recipes)

data <- data.frame(a = c(LETTERS, NA))

aaa <- recipe(~ a, data = data) %>%
  step_dummy(a) %>%
  prep()
#> Warning: ! There are new levels in `a`: NA.
#> ℹ Consider using `recipes::step_unknown()` before `step_dummy()` to handle
#>   missing values.

data_train <- tibble(x = c("a", "b"))
data_test <- tibble(x = "c")

rec_spec <- recipe(~., data = data_train) %>%
  step_dummy(x) %>%
  prep()

aaa <- rec_spec %>%
  bake(data_test)
#> Warning: ! There are new levels in `x`: "c".
#> ℹ Consider using `recipes::step_novel()` before `step_dummy()` to handle unseen
#>   values.

[simonpcouch]

This is awesome. Knocking out all sorts of QOL improvements. :)

[EmilHvitfeldt]

the new snapshots looks worse, but they print neater!

[github-actions]

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