guide_coloursteps(even.steps = FALSE) works with discrete data

[teunbrand]

This PR aims to fix #3877.

Briefly, when binning is done outside the scale with cut(), the coloursteps guide understood how to display this, but couldn't display this with uneven steps. This PR makes the guide work with uneven steps.

Reprex from the linked issue:

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

mtcars |>
  transform(mpg_cut = cut(mpg, c(seq(10, 25, 5), 35))) |>
  ggplot(aes(hp, mpg, color = mpg_cut)) +
  geom_point() +
  guides(color = guide_colorsteps(show.limits = TRUE, even.steps = FALSE))

^{Created on 2024-03-19 with reprex v2.1.0}

@thomasp85 mentioned why this was unsupported before here, but I didn't figure out why it was unsupported.

[thomasp85]

Well, tbh I can't remember the reason either but it appears I was right in that it had to wait for the guide rewrite 😄