Inheritance of theme-based aesthetics

[teunbrand]

This PR is an amendment to #5833.

Briefly, we want to deprecate update_geom_defaults() and to do that we need to increase the flexibility of the theme based styling of geom defaults. In this PR, we include inheritance of theme-based aesthetics to allow finer customisation.

In order to make this not too restrictive, we're allowing any theme(geom.* = element_geom()), which takes same logic in terms of theme validation.

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

class(GeomDensity)
#> [1] "GeomDensity" "GeomArea"    "GeomRibbon"  "Geom"        "ggproto"    
#> [6] "gg"

ggplot(mpg, aes(displ)) +
  geom_density() +
  theme(
    geom.ribbon = element_geom(borderwidth = 2),
    geom.area = element_geom(bordertype = 2),
    geom.density = element_geom(ink = "red")
  )

^{Created on 2025-01-21 with reprex v2.1.1}