Add col_contrasting()

[arcresu]

This function is very useful for picking an appropriate text colour based on the background colour, especially inside after_scale(). The intended usage is this:

library(ggplot2)
library(dplyr)

expand.grid(x = 1:5, y = 1:5) |>
  mutate(n = row_number()) |>
  ggplot(aes(x, y, fill = n)) +
  geom_tile()+
  geom_text(aes(
    label = n,
    colour = after_scale(col_contrasting(fill))
  ))

There are many versions of this function floating around on the internet, many using a lot of heuristics to find the threshold for light/dark. I went with using the midpoint of lightness in Lab space. Of course this could be made configurable, and it doesn't necessarily have to live alongside the colour manipulation family. I do think it's worth having something like this in scales though.