Arrow key size

[teunbrand]

This PR implements #5465 (comment) for arrow-based keys.

Briefly, it allows arrows to allocate legend key space for themselves.

I think a visual demonstrates the effect best.
We go from this at current main branch:

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

ggplot(mapping = aes(x = 0, xend = 1)) +
  geom_segment(
    aes(y = 0, yend = 0, colour = "arrow 1"),
    arrow = arrow(length = unit(1, "cm"), angle = 10)
  ) +
  geom_segment(
    aes(y = 1, yend = 1, colour = "arrow 2"),
    arrow = arrow(length = unit(1, "cm"), angle = 70, ends = "both")
  ) +
  geom_segment(
    aes(y = 2, yend = 2, colour = "arrow 3"),
    arrow = arrow(length = unit(1, "cm"), angle = 45, ends = "first")
  )

^{Created on 2023-12-04 with reprex v2.0.2}

To the following with this PR: