Add option to `facet_grid2()` to snap axes to panels

[teunbrand]

Inspo: https://blog.djnavarro.net/posts/2025-06-03_ggplot2-scatterplot-pairs/

library(ggh4x)
#> Loading required package: ggplot2

df <- data.frame(
  x = 1, y = 1,
  xvar = c("A", "B", "B", "C", "C", "C"),
  yvar = c("1", "1", "2", "1", "2", "3")
)

ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  facet_grid2(yvar ~ xvar, render_empty = FALSE)

^{Created on 2025-09-08 with reprex v2.1.1}