Only sort numeric breaks in binned guides

[teunbrand]

This PR aims to fix #5757.

Briefly, guides try to parse bins from the break information in a scale. To fix #5152, we automatically sorted the breaks. However, that should only happen for numeric breaks. This PR moves the sorting step to apply to numeric breaks only.

Reprex from issue:

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

ggplot(mtcars, aes(mpg, drat, colour = cut(disp, c(90, 200, 300, 400, 500)))) +
  geom_point() +
  scale_colour_viridis_d(guide = guide_colorsteps())
#> Warning: Removed 4 rows containing missing values or values outside the scale range
#> (`geom_point()`).

^{Created on 2024-03-07 with reprex v2.1.0}

[thomasp85]

LGTM