Feature Request: Minor breaks for discrete position scales

[teunbrand]

In discrete scales, the breaks are centered on groups, which can sometimes make it difficult to visually discern where groups start and end. To add more visual cues about this, I think it is a good idea to add a minor.breaks argument to discrete position scales, that subsequently puts minor panel grid lines at those positions. I also thinks it may pair well with #5287. Essentially, I'd like to have the following visual:

library(ggplot2)
#> Warning: package 'ggplot2' was built under R version 4.3.1

ggplot(mpg, aes(class, fill = factor(cyl))) +
  geom_bar(position = position_dodge(preserve = "single")) +
  annotate(
    "segment",
    x = 1.5:6.5, xend = 1.5:6.5,
    y = -Inf, yend = Inf,
    colour = 'white'
  ) +
  theme(panel.grid.major = element_blank())

But with this syntax:

ggplot(mpg, aes(class, fill = factor(cyl))) +
  geom_bar(position = position_dodge(preserve = "single")) +
  scale_x_discrete(minor.breaks = 1.5:6.5) +
  theme(panel.grid.major = element_blank())

^{Created on 2023-09-22 with reprex v2.0.2}