New widthDetails() and heightDetails() methods for plots

[teunbrand]

This PR aims to fix #6367.

It implements widthDetails() and heightDetails() methods, that essentially convert a plot to a gtable and use the gtable method.

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

widthDetails(p)
#> [1] sum(5.5points, 0points, 0cm, 0points, 1null, 1null, 0cm, 0cm, 0points, 0cm, 0points, 5.5points, 0cm, 1null, 0.447737823439878cm, 0null, max(0points, -0.0966514459665145cm), 0.0966514459665145cm)

heightDetails(p)
#> [1] sum(5.5points, 0points, 0cm, 0cm, 0cm, 0points, 0cm, 0cm, 1null, 1null, 0points, 0cm, 0cm, 0points, 5.5points, 0.0966514459665145cm, max(0points, -0.0966514459665145cm), 0null, 0.394821156773212cm, 1null, 0cm)

^{Created on 2025-03-21 with reprex v2.1.1}

[thomasp85]

I'm a bit hesitant on this since these generics are explicitly intended for grobs, which ggplot and ggplot_build objects are not