[Accessibility] Allow alt to take a function

[jooyoungseo]

CC @hadley

Currently, there is no easy way to pass a function return to alt argument using dot symbol. Please allow alt to take a function return.

Reprex

library(ggplot2)

ggplot(mpg, aes(class)) +
  geom_bar() +
  labs(alt = BrailleR::VI(.)$text)
#> Error in BrailleR::VI(.): object '.' not found

^{Created on 2022-04-04 by the reprex package (v2.0.1)}

Session info

sessioninfo::session_info()
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#>  date     2022-04-04
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#> ------------------------------------------------------------------------------