Make facet section of summary() more terse

[teunbrand]

This PR aims to fix #5989.

Briefly, summary.ggplot() no longer prints the whole facet ggproto object, but rather prints the variables used for facetting.

Some examples with various facets:

devtools::load_all("~/packages/ggplot2/")
#> ℹ Loading ggplot2
p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point()

summary(p)
#> data: manufacturer, model, displ, year, cyl, trans, drv, cty, hwy, fl,
#>   class [234x11]
#> mapping:  x = ~displ, y = ~hwy
#> faceting:  <empty> 
#> -----------------------------------
#> geom_point: na.rm = FALSE
#> stat_identity: na.rm = FALSE
#> position_identity

summary(p + facet_wrap(~drv))
#> data: manufacturer, model, displ, year, cyl, trans, drv, cty, hwy, fl,
#>   class [234x11]
#> mapping:  x = ~displ, y = ~hwy
#> faceting:  ~drv 
#> -----------------------------------
#> geom_point: na.rm = FALSE
#> stat_identity: na.rm = FALSE
#> position_identity

summary(p + facet_grid(drv ~ year))
#> data: manufacturer, model, displ, year, cyl, trans, drv, cty, hwy, fl,
#>   class [234x11]
#> mapping:  x = ~displ, y = ~hwy
#> faceting:  ~drv, ~year 
#> -----------------------------------
#> geom_point: na.rm = FALSE
#> stat_identity: na.rm = FALSE
#> position_identity

# Reasonable results for extension facets
summary(p + ggforce::facet_matrix(~drv))
#> data: manufacturer, model, displ, year, cyl, trans, drv, cty, hwy, fl,
#>   class [234x11]
#> mapping:  x = ~displ, y = ~hwy
#> faceting:  ~row_data, ~col_data 
#> -----------------------------------
#> geom_point: na.rm = FALSE
#> stat_identity: na.rm = FALSE
#> position_identity

^{Created on 2024-09-04 with reprex v2.1.1}

[yutannihilation]

Looks good to me.

[teunbrand]

Thanks for the review!