fix draw_key for GeomVector

[corybrunson]

Currently the vector geom defaults to a marker (point) key rather than a segment/arrow key, which makes the legends illegible:

library(gggda)
#> Loading required package: ggplot2

ability.cov$cov |> 
  princomp() |> 
  getElement("loadings") |> 
  unclass() |> as.data.frame() |> tibble::rownames_to_column(var = "scale") |> 
  transform(type = ifelse(scale == "general", "composite", "primary")) |> 
  ggplot(aes(Comp.1, Comp.2)) +
  coord_equal() +
  geom_vector(aes(linetype = type, label = scale))

^{Created on 2025-10-02 with reprex v2.1.1}