Warn about simultaneous use of mapped and static aesthetics

[teunbrand]

This PR aims to fix #6429.

Instead of improving the 'no shared levels' warning as implied by the issue, this PR separately warns about the simulatenous use of an aesthetic in mapping and as a fixed aesthetic.

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(size = cty), size = 12)
#> Warning in geom_point(aes(size = cty), size = 12): The size aesthetic is defined twice: once in `mapping` and once as a static
#> aesthetic.
#> ℹ The static aesthetic overrules the mapped aesthetic.

We only do this when this occurs within a layer. If local fixed aesthetics contradict the global mapping, this is not a warning:

# silent
p <- ggplot(mpg, aes(displ, hwy, size = cty)) +
  geom_point(size = 12)

^{Created on 2025-11-12 with reprex v2.1.1}

This is because people can intentionally deviate the local aesthetic from global mappings to exempt that aesthetic from inherit.aes. In contrast, I don't think you can intentially have contradicting aesthetics in a single layer.