Add format() method for class_gg

[teunbrand]

This PR aims to fix #6526.

This fix works only in R-devel, as it relies on r-devel/r-svn@940b5c2.
I don't think it is possible to also fix this for release/old versions.
However, it is probably harmless to have a format method around, regardless of R version.

Quick demonstration:

getRversion()
#> [1] '4.6.0'
devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2

z <- iris
z$gg <- list(ggplot())
print(head(z))
#>   Sepal.Length Sepal.Width Petal.Length Petal.Width Species                gg
#> 1          5.1         3.5          1.4         0.2  setosa <ggplot2::ggplot>
#> 2          4.9         3.0          1.4         0.2  setosa <ggplot2::ggplot>
#> 3          4.7         3.2          1.3         0.2  setosa <ggplot2::ggplot>
#> 4          4.6         3.1          1.5         0.2  setosa <ggplot2::ggplot>
#> 5          5.0         3.6          1.4         0.2  setosa <ggplot2::ggplot>
#> 6          5.4         3.9          1.7         0.4  setosa <ggplot2::ggplot>

^{Created on 2025-09-26 with reprex v2.1.1}