Ignoring AsIs objects

[teunbrand]

This PR aims to fix #5142.

Briefly, all scales ignore AsIs objects instead of invoking an identity scale.

Contrary to my expectation, I only had to edit ScalesList methods (and their position equivalents) and no individual scales, so no deep rewrite was needed.
While in #5142 I expressed worry about how position aesthetics would play with this, I've grown to love ignored position aesthetics. No more annotation_custom() or awkward manual transformations if you want to make relative annotations!

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

df <- data.frame(t = seq(0, 2*pi, length.out = 100))

ggplot(mpg) +
  geom_point(aes(displ, hwy)) +
  geom_path(
    data = df, 
    aes(x = I(cos(t) / 2.1 + 0.5), 
        y = I(sin(t) / 2.1 + 0.5),
        colour = I(2))
  ) +
  annotate(
    "text", x = I(0.5), y = I(0.5),
    label = "I'm in the middle",
    colour = 4, size = 10
  )

^{Created on 2023-04-26 with reprex v2.0.2}

[thomasp85]

As discussed we should investigate if we could instead using the same mechanics as after_scale() for this functionality

[teunbrand]

Closing this in favour of #5477