Function format_document 

pub fn format_document(
    text: &str,
    config: &FormatConfig,
) -> Result<String, String>

Parse an FD document, apply configured transforms, and re-emit canonical text.

The output is idempotent: format_document(format_document(s, c), c) == format_document(s, c).

Errors

Returns the parser error string if the input is not valid FD syntax.