performance bug with `engine = "lightgbm"`

[simonpcouch]

Noticed while working on emlwr the other day that bonsai::train_lightgbm() is quite a bit slower than lightgbm::lgb.train(), probably due to the handling of categorical variables / conversion to lgb.Dataset. Observed with emlwr:::simulate_classification()—@EmilHvitfeldt also noted a slowdown from a user with a similar-appearing dataset last week.