parse method
Implementation
@override
Node? parse(BlockParser parser) {
final current = parser.current.content;
final pos = parser.pos + parser.offset;
final match = pattern.firstMatch(current)!;
final label = match[2]!;
final refs = parser.document.footnoteReferences;
refs[label] = 0;
final id = Uri.encodeComponent(label);
parser.advance();
final lines = [
Line(current.substring(match[0]!.length)),
...parseChildLines(parser),
];
final children = parser.document.getBlockParser(lines,
offset: pos).parseLines();
return Element('li', children)
..attributes['id'] = 'fn-$id'
..footnoteLabel = label;
}