Add alt text to `autoplot` methods

[juliasilge]

To improve accessibility, we can add alt labels to the output of the autoplot methods.

Here is a very rough idea of a possibility:

library(tidymodels)
#> Registered S3 method overwritten by 'tune':
#>   method                   from   
#>   required_pkgs.model_spec parsnip
data("hpc_cv")

# The confusion matrix from a single assessment set (i.e. fold)
cm <- hpc_cv %>%
  filter(Resample == "Fold01") %>%
  conf_mat(obs, pred)

autoplot(cm, type = "mosaic") +
  labs(alt = paste0(
    "A mosaic confusion matrix for classes VF, F, M, and L. ",
    "The largest cell is VF/VF."
  ))

^{Created on 2021-05-19 by the reprex package (v2.0.0)}

We can do more learning about how to write alt text for this context.