Allow expression labels in scale_{x,y}_*()

[yutannihilation]

Fix #6617

This function size0 was introduced in #6076 and I guess the expression labels don't work since then. Reading #6076, I found there's no intention to drop support for expression labels, so I think this fixes the problem. (As I don't actively follow the recent development of ggplot2, sorry if I'm missing some discussion)

library(tibble)
devtools::load_all("~/GitHub/ggplot2/")
#> ℹ Loading ggplot2
#> Overwriting method +(<ggplot2::ggplot>, <ANY>)
#> 
#> Overwriting method +(<ggplot2::theme>, <ANY>)
#> 
#> Overwriting method convert(<ggplot2::ggplot>, <list>)

ggplot(tibble(x = -10:10, y = x^2)) +
  geom_point(aes(x, y)) +
  scale_x_continuous(
    breaks = seq(-10, 10, 2),
    labels = parse(text = paste0(seq(-10, 10, 2), "^degree"))
  )

^{Created on 2025-09-21 with reprex v2.1.1}

[teunbrand]

Great, thank you! If you want we could add a news bullet to indicate the issue is fixed.

[yutannihilation]

Thanks! Added a news bullet.

[teunbrand]

Thanks! I'd be happy to merge it, but you can also do this yourself I believe

[yutannihilation]

Then, I'm merging. Thanks for reviewing!