Warn about ignored scales and guides

[teunbrand]

This PR aims to fix #3743 and guides indentified in https://github.com/tidyverse/ggplot2/issues/3196#issuecomment-2270818015.

Briefly, when using a scale or guide for an aesthetic that is not in use, you'll get a warning.

Implementation-wise, it wasn't too straightforward to allow guides and scales to assess their usage. A few remarks on this in the code.

An example:

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

ggplot(mpg, aes(displ, hwy, colour = drv)) +
  geom_point() +
  scale_fill_discrete() +
  guides(shape = "bar")
#> Warning: Ignoring unknown guide: "shape".
#> Warning: Ignoring scale for unused aesthetics: "fill".

^{Created on 2024-08-07 with reprex v2.1.1}