Fix problem with arrow lengths in glyph drawing

[teunbrand]

This PR aims to fix #6594.

Briefly, it uses the length of the first arrow (rather than all arrows) to determine key glyph size.
The key glyph only ever draws the first arrow.

In addition, this PR also provides arrow support for line-ish keys:

• draw_key_abline
• draw_key_vline
• draw_key_timeseries

Reprex from issue:

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

segment_df <- data.frame(
  x = c(1, 2, 3),
  y = c(1, 2, 1),
  xend = c(2, 3, 4),
  yend = c(2, 1, 2),
  group = c("A", "B", "C"),
  arrow_length = c(0.1, 0.2, 0.3)
)
ggplot(segment_df) +
  geom_segment(
    aes(x = x, y = y, xend = xend, yend = yend, color = group),
    arrow = arrow(length = unit(segment_df$arrow_length, "inches"), type = "closed", angle = 30),
    linewidth = 1.5
  ) +
  theme_minimal()

^{Created on 2025-09-08 with reprex v2.1.1}

[thomasp85]

LGTM