Discrete labels from break names

[teunbrand]

This PR aims to fix #6147 and fix #6089.

Briefly, names(breaks) is used as default label for discrete breaks. We also use this mechanism to populate discrete secondary labels (#6089).

Reprex from #6147 with a secondary axis:

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

ggplot(mpg, aes(drv, hwy)) +
  geom_boxplot() +
  scale_y_continuous(
    breaks = c(twenty = 20, thirty = 30, fourty = 40)
  ) +
  scale_x_discrete(
    breaks = c("four-wheel" = 4, forward = "f", reverse = "r"),
    sec.axis = dup_axis()
  )

^{Created on 2024-10-18 with reprex v2.1.1}