Size of text/label keys

[teunbrand]

This PR aims to fix #5561.

Briefly, it uses the mechanism introduced in #5465 to communicate the proper width and height of the grobs.
As a detail, since the labelGrob uses the rotation through the viewport, size has to be calculated after manual rotation.

Reprex from #5561:

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

ggplot(mpg, aes(displ, hwy, label = class, colour = class)) +
  geom_text(angle = 45) +
  discrete_scale(
    "label", palette = identity_pal(), super = ScaleDiscreteIdentity
  )

ggplot(mpg, aes(displ, hwy, label = class, colour = class)) +
  geom_label(angle = -45) +
  discrete_scale(
    "label", palette = identity_pal(), super = ScaleDiscreteIdentity
  )

^{Created on 2023-12-04 with reprex v2.0.2}

[teunbrand]

Selfishling adding this to the milestone because I like to use this myself.

[thomasp85]

LGTM

[teunbrand]

Thanks for the review!