after_*() length checks

[teunbrand]

This PR aims to fix #5901.

Briefly, when after_stat() or after_scale() produce aesthetics of the wrong length, a warning is thrown that the function could not be applied.

The reprex from the issue now renders with a warning:

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

ggplot(mpg[15:16, ], aes(displ, hwy)) +
  geom_point(
    aes(colour = after_scale(c("red", "green", "blue")))
  )
#> Warning: Failed to apply `after_scale()` for the following aesthetic: colour.

^{Created on 2024-05-22 with reprex v2.1.0}

I have an alternate agenda with this PR.
Moving this check into Geom$use_defaults() frees us from having to do so in the legend code, allowing a simplification I ultimately need for #5833.

[thomasp85]

LGTM

[teunbrand]

Thanks for the review Thomas!