Continuous limits for discrete scales

[teunbrand]

This PR aims to fix #6259 and fix #4174.

Briefly, the discrete position scales get a new argument continuous.limits which controls what range is displayed.
It always acts as 'zoom limits', similar to limits set in the coord or continuous limits with oob = scales::oob_keep().

Reprex from #4174:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
df <- data.frame(
  stringsAsFactors = FALSE,
  continent = c("europe","europe","europe",
                "europe","asia","asia","asia","south america"),
  country = c("AT", "DE", "DK", "ES", "PK", "IN", "TW", "BR"),
  value = c(10L, 15L, 20L, 25L, 17L, 17L, 13L, 5L)
)

p <- df |>
  ggplot() +
  geom_col(
    aes(y = country, x = value),
    orientation = "y"
  ) +
  facet_wrap(vars(continent), scales = "free_y")

p + scale_y_discrete(continuous.limits = c(1, 4))

^{Created on 2025-01-07 with reprex v2.1.1}

[teunbrand]

I now also believe this would be needed for #3441 to make copies of discrete scales with immutable ranges.