Alt text label can be function

[teunbrand]

This PR aims to fix #4795, by allowing alt to be a function that takes the plot as input.

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

p <- ggplot(mpg, aes(class)) +
  geom_bar() +
  labs(alt = ~ BrailleR::VI(.x)$text)

cat(get_alt_text(p), sep = "\n")
#> This is an untitled chart with no subtitle or caption.
#> It has x-axis 'class' with labels 2seater, compact, midsize, minivan, pickup, subcompact and suv.
#> It has y-axis 'count' with labels 0, 20, 40 and 60.
#> The chart is a bar chart with 7 vertical bars.
#> Bar 1 is centered horizontally at 2seater, and spans vertically from 0 to 5.
#> Bar 2 is centered horizontally at compact, and spans vertically from 0 to 47.
#> Bar 3 is centered horizontally at midsize, and spans vertically from 0 to 41.
#> Bar 4 is centered horizontally at minivan, and spans vertically from 0 to 11.
#> Bar 5 is centered horizontally at pickup, and spans vertically from 0 to 33.
#> Bar 6 is centered horizontally at subcompact, and spans vertically from 0 to 35.
#> Bar 7 is centered horizontally at suv, and spans vertically from 0 to 62.

^{Created on 2022-12-03 by the reprex package (v2.0.1)}

[thomasp85]

LGTM