parse method
Implementation
@override
Node parse(BlockParser parser) {
  final childLines = parseChildLines(parser);
  var text = childLines.map((e) => e.content).join('\n').trimRight();
  if (parser.previousSyntax != null || parser.parentSyntax != null) {
    text = '\n$text';
    if (parser.parentSyntax is ListSyntax) {
      text = '$text\n';
    }
  }
  return Text(text);
}