update partykit_tree_info() to handle classification outputs

[EmilHvitfeldt]

Left todo:

• add tests
• make sure the stat_mode() helper does what partykit would have done (i doubt it does right now)

[EmilHvitfeldt]

example

library(parsnip)
library(bonsai)
library(tidypredict)

tree_cls <- decision_tree("classification", "partykit")

fit(tree_cls, Species ~., iris) |>
  tidypredict_fit()
#> case_when(Petal.Length <= 1.9 ~ "versicolor", Petal.Length <= 
#>     4.8 & Petal.Width <= 1.7 & Petal.Length > 1.9 ~ "setosa", 
#>     Petal.Length > 4.8 & Petal.Width <= 1.7 & Petal.Length > 
#>         1.9 ~ "setosa", Petal.Width > 1.7 & Petal.Length > 1.9 ~ 
#>         "setosa")

^{Created on 2024-11-20 with reprex v2.1.0}