Make position_dodge2() work for geom_violin()

[teunbrand]

This PR aims to fix #6663.

It has the fix to treat multi-row groups correctly, by dodging only the first row of each group and then reinserting the results over multiple rows.
Notably, it does not have fancy detection of when this should apply.
It is currently controlled by position_dodge2(group_row) but ideally, this would be detected automatically.

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

ggridges::Aus_athletes |>
  ggplot(aes(sport, height, fill = sex)) +
  geom_violin(position = position_dodge2(group_row = "many"))

^{Created on 2025-09-30 with reprex v2.1.1}

[thomasp85]

Just because dodging is so finicky it would be nice with some unit tests to cover this