Support after_stat(), after_scale(), and stage()

[Hiroaki Yutani]

The current code considers only stat() and ..var..-notation, but there's more to come in ggplot2 3.3.0.

gghighlight/tests/testthat/test-calculate-group.R

Lines 30 to 33 in 24aa705

 # if aes contains expressions that cannot be evaluated outside ggplot2 (e.g. stat(count)), 
 # just ignore it. 
 expect_equal(calculate_group_info(d, aes(x, y, colour = type, fill = stat(count), alpha = ..count..)), 
              list(data = d_expect, id = ids, key = aes(colour = type))) 

[Hiroaki Yutani]

This is already supported (the mechanism doesn't assume the specific names as stat())