Move footer function to cnd_footer() method

[DavisVaughan]

Closes #211

I've shifted us from using an on-the-fly created footer function to a cnd_footer() S3 method. This forces us to retain the minimal amount of things required to reproduce the footer message inside the cnd object, which seems to be the pkg name of interest and always.

We've got quite a few snapshot tests that test the emitted warning messages, so I feel pretty good about this.

library(dplyr)

tibble(
  x = 1:1000
) |> 
  cross_join(
    tibble(
      y = 1:1000
    )
  ) |> 
  mutate(
    v0 = "y"
  ) |> 
  mutate(
    v1 = case_match(v0, "blah" ~ "y")
  )

# BEFORE
lobstr::obj_size(dplyr:::the$last_warnings[[1]]$cnd)
#> 24.20 MB

# AFTER
lobstr::obj_size(dplyr:::the$last_warnings[[1]]$cnd)
#> 22.64 kB

[DavisVaughan]

I was a bit worried about the fact that trace is in the condition object, but it doesn't seem to be that big, so it's probably okay.

[DavisVaughan]

We weren't using internal for anything