POC: replace facet_grid(switch) with strip.position

[teunbrand]

This PR aims to fix #5999.

Briefly, it deprecates facet_grid(switch) in favour of facet_grid(strip.position = "top"/"bottom"/"left"/"right").

If one of the directions is omitted (top/bottom in example below), the default will be filled in:

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

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

p + facet_grid(
  drv ~ year,
  strip.position = "left"
)

However, when specifying >1 position in the same direction, this taken literally.

p + facet_grid(
  drv ~ year,
  strip.position = c("top", "bottom", "left", "right")
)

^{Created on 2024-10-25 with reprex v2.1.1}

It is currently a POC because I'm unsure of whether we should do this. Reverse dependency checks revealed a lot of facet extensions that are rather sensitive to parameters in facet_grid()/facet_wrap() when implementing #5855. For this reason, I expect some revdep troubles with this PR. In contrast to #5855, which adds new functionality, this PR is mostly 'cosmetic'.