Coord radial panel margins

[Yunuuuu]

Fix #6379

This PR provides panel.margin argument to control the spacing between CoordRatial panel and axis

[Yunuuuu]

ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  coord_radial(
    start = -0.4 * pi,
    end = 0.4 * pi
  )

ggplot(mtcars, aes(disp, mpg)) +
  geom_point() +
  coord_radial(
    start = -0.4 * pi,
    end = 0.4 * pi,
    panel.margin = unit(0, "mm")
  )

[Yunuuuu]

It seems it's easier to implement outer.radius instead of this spacing.