fix(ellmer_prompt): Truncate lines and items individually

[gadenbuie]

Fixes a small bug in print.ellmer_prompt() to correctly truncate lines and items independently.

I noticed this by using ellmer::interpolate_file() to read in a prompt with >200 lines.

Before

ellmer::interpolate(paste(letters, collapse = "\n")) |> 
  print(max_lines = 3)
#> [1] │ a
#>     │ b
#>     │ c
#>     │ ...
#> Error in if (n_extra > 0) {: missing value where TRUE/FALSE needed

After

ellmer::interpolate(paste(letters, collapse = "\n")) |> 
  print(max_lines = 3)
#> [1] │ a
#>     │ b
#>     │ c
#>     │ ...and 23 more lines.

[hadley]

I just fixed this in testthat too: r-lib/testthat#2257. You might want to take that instead since it looks a little simpler.

This is why you shouldn't copy and paste code kids.