Discrete minor ticks

[teunbrand]

This PR aims to fix #5434.

Briefly, it adds a minor_ticks argument to discrete scales, which I imagine is only useful for position scales.
It controls where minor panel grid lines and minor tick marks are placed, so you can do things like the following:

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

ggplot(mpg, aes(class, fill = factor(cyl))) +
  geom_bar(position = position_dodge(preserve = "single")) +
  scale_x_discrete(
    minor_breaks = breaks_width(1, 0.5),
    guide = guide_axis(minor.ticks = TRUE)
  ) +
  theme(
    panel.grid.major.x = element_blank(),
    axis.minor.ticks.length.x.bottom = unit(11, "pt"),
    axis.ticks.length.x.bottom = unit(0, "pt"),
    axis.text.x.bottom = element_text(margin = margin(t = -8.75))
  )

Because the minor breaks are computed on the mapped breaks/limits, rather than on limits, they don't have compatible .value columns that can be combined with discrete .value columns, so the guide drops such incompatible values.

get_guide_data(aesthetic = "x")
#>             x     .value     .label .type y
#> 1  0.08333333    2seater    2seater major 0
#> 2  0.22222222    compact    compact major 0
#> 3  0.36111111    midsize    midsize major 0
#> 4  0.50000000    minivan    minivan major 0
#> 5  0.63888889     pickup     pickup major 0
#> 6  0.77777778 subcompact subcompact major 0
#> 7  0.91666667        suv        suv major 0
#> 8  0.01388889       <NA>       <NA> minor 0
#> 9  0.15277778       <NA>       <NA> minor 0
#> 10 0.29166667       <NA>       <NA> minor 0
#> 11 0.43055556       <NA>       <NA> minor 0
#> 12 0.56944444       <NA>       <NA> minor 0
#> 13 0.70833333       <NA>       <NA> minor 0
#> 14 0.84722222       <NA>       <NA> minor 0
#> 15 0.98611111       <NA>       <NA> minor 0

^{Created on 2024-03-15 with reprex v2.1.0}

As a small note, the minor_breaks argument uses snake_case for consistency with continuous scales rather than dot.case.

[thomasp85]

LGTM