formatter should rename foo_barBaz to foo_bar_baz

[JanCVanB]

roc-lang/basic-cli#321 was a manual reformat of epoch_millis_to_datetimeHelp (which was added as-is) to the fully snake-cased epoch_millis_to_datetime_help. It would be nice for the formatter to eliminate all uppercase characters in def names, even if there's already one or more underscores in the name. I have no context for whether this seems easy or hard. Thanks!