add appropriate default marker for arrows legend entry

[japhir]

Feature description

The default legend marker for the arrows function doesn't seem to work as I would expect:

In 2d it shows a line with a triangle in the middle

while in 3d it doesn't show any symbol

@jkrumbiegel mentioned on slack that:
it just falls back to lines + scatter legend
so nothing special was implemented for either case

MWE:

using GLMakie
fig = Figure()
ax = Axis(fig[1, 1]) 
a = arrows!(ax, [0],[0], [1],[1], label = "Hi there")
axislegend(position = :lt)

fig = Figure()
ax = Axis3(fig[1, 1], aspect = :data) 
o = Point3f(0.0, 0.0, 0.0)
n = Vec3f(0, 0, 1)
a = arrows!(ax,[o], [n], label = "Hi there")
a.plots[1].transform_marker = true
a.plots[2].transform_marker = true
axislegend(position = :lt)

For plot types, please add an image of how it should look like

for 2d: a small arrow? probably at 45° or so?

for 3d: a small 3d arrow? probably at 45° or so?