parse method
Implementation
@override
Node? parse(BlockParser parser) {
final lines = parser.linesToConsume;
if (lines.length < 2) {
return null;
}
// Remove the last line which is a marker.
lines.removeLast();
final marker = parser.current.content.trim();
final level = (marker[0] == '=') ? '1' : '2';
final content = lines.map((e) => e.content).join('\n').trimRight();
parser.advance();
return Element('h$level', [UnparsedContent(content)]);
}