Polygon/timeseries legend keys reflect outline.type setting

[teunbrand]

This PR aims to fix #6649.

This PR adds additional lines to the key glyphs when the outline.type is not full.
Arguably draw_key_timeseries() could also be made to support the fill aesthetic under all circumstances, but that is not enforced in this PR.

Demonstrations, notice the legend keys:

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

ggplot(economics, aes(date)) +
  geom_area(
    aes(y = unemploy, fill = "Unemployment"),
    alpha = 0.5, colour = "black", outline.type = "upper"
  ) +
  geom_area(
    aes(y = pce, fill = "PCE"),
    alpha = 0.5, colour = "black", outline.type = "both"
  ) +
  geom_area(
    aes(y = pop / 30, fill = "Population"),
    alpha = 0.5, colour = "black", outline.type = "lower"
  ) +
  theme(
    legend.key.spacing.y = unit(5.5, "pt")
  )

And for draw_key_timeseries():

ggplot(economics, aes(date)) +
  geom_area(
    aes(y = unemploy, fill = "Unemployment"),
    alpha = 0.5, colour = "black", outline.type = "upper",
    key_glyph = draw_key_timeseries
  ) +
  geom_area(
    aes(y = pce, fill = "PCE"),
    alpha = 0.5, colour = "black", outline.type = "both",
    key_glyph = draw_key_timeseries
  ) +
  geom_area(
    aes(y = pop / 30, fill = "Population"),
    alpha = 0.5, colour = "black", outline.type = "lower",
    key_glyph = draw_key_timeseries
  ) +
  theme(
    legend.key.spacing.y = unit(5.5, "pt")
  )

^{Created on 2025-09-25 with reprex v2.1.1}