Proof-of concept support for unit() throughout ggplot2

[mjskay]

This PR is a proof-of-concept for support of unit() throughout ggplot2. It shows how some of the ideas in #5609 might be implemented.

To tease, it would allow you to do stuff like this:

data.frame(var1 = 1:5, var2 = 1:5, name = letters[1:5]) |>
  ggplot(aes(var1, var2)) + 
  geom_point() +
  geom_line() +
  # labels exactly 10 points right of their points, no matter how the
  # plot is resized
  geom_text(aes(label = name, x = var1 + as_pt(10))) +
  # an annotation that is always 10 points inset from the lower right
  annotate("text", 
    x = as_npc(1) - as_pt(10), 
    y = as_pt(10), 
    label = "some label", vjust = 0, hjust = 1
  ) +
  coord_polar()

It is a draft only for several reasons:

1. Several of the proposed changes probably shouldn't live in ggplot2, but in vctrs, grid, or perhaps scales.
2. This is just a prototype, not a comprehensive, solid, well-tested solution.
3. While the solution for positional aesthetics (I think) covers most cases, the solution for non-positional aesthetics is only mocked up for geom_point().

Would love to hear folks' thoughts, or if I am barking up the wrong tree...