transform method
Implementation
@override
String transform(Node node, DocumentMarkdownEncoder? encoder) {
final delta = node.delta ?? Delta()
..insert('');
final markdown = DeltaMarkdownEncoder().convert(delta);
final attributes = node.attributes;
final level = attributes[HeadingBlockKeys.level] as int? ?? 1;
final result = '${'#' * level} $markdown';
final suffix = node.next == null ? '' : '\n';
return '$result$suffix';
}