Driver that writes the syntax tree to a string by visiting tokens.
You can use [format] directly, or build a custom flow with walk(root, &mut driver, &opts).
Print the syntax tree to a string.
When canonical_format, parenthesize_expressions, or annotate_types is set, runs a custom walk;
otherwise uses sipha’s emit API for round-trip.
Try to load formatter options from a directory: first .leekfmt.toml, then leekscript.toml with [format] section.
Returns None if no file is found or parsing fails; caller falls back to FormatterOptions::default().