✨ Justification for legend keys

[teunbrand]

This PR aims to fix #3669.

It introduces a new theme element legend.key.justification to control the placement of legend keys when there is more space available than needed to display the key. This condition can be met by having text that is larger than the key or when keys have unequal sizes like in linewidth or size legends. I arrived at the conclusion that this should be a justification setting because it was unclear where exactly to place the key if it were just a boolean setting. Moreover, this nicely allows for justification settings in both the horizontal and vertical direction (see snapshot test).

Reprex from the issue, note that the keys in the the second row of the first legend no longer stretch:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
set.seed(15)

df <- data.frame(x=1:50, y=rnorm(50, 10, 2), var=rep(c("A","B","C","D","E"),10))
labs = c("A oneline","B oneline","C oneline", "D\ntwolines","E\ntwolines")
cols = c('#e41a1c','#377eb8','#4daf4a','#984ea3','#ff7f00')


ggplot(df) + 
  geom_ribbon(aes(x=x, ymin=y-1, ymax=y+1, fill=var), alpha=0.3) +
  geom_line(aes(x=x, y=y, colour=var), alpha=0.3, linewidth=2) +
  scale_fill_manual(values=cols, labels = labs) +
  theme(legend.position="bottom",
        legend.title = element_blank(),
        legend.text = element_text(size = 14)) +
  guides(fill=guide_legend(ncol=3, byrow=TRUE)) +
  theme(legend.key.justification = "top")

^{Created on 2025-01-10 with reprex v2.1.1}