Fix label formatting bug in warning

[teunbrand]

This PR aims to fix #6463.

Briefly, function-labels are formatted as <function> so they don't mess with text formatting.
Example from reprex, note the warning about unknown labels:

library(palmerpenguins)
devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

penguins |> 
  ggplot() +
  geom_histogram(aes(x = flipper_length_mm)) +
  labs(
    x = snakecase::to_sentence_case,
    y = snakecase::to_sentence_case,
    fill = snakecase::to_sentence_case,
  )
#> Warning: Ignoring unknown labels:
#> • fill : <function>
#> `stat_bin()` using `bins = 30`. Pick better value `binwidth`.
#> Warning: Removed 2 rows containing non-finite outside the scale range
#> (`stat_bin()`).

^{Created on 2025-05-28 with reprex v2.1.1}