How to make Flexmark-Java process "@"-mentions?

When processing markdown, GitHub supports the @-syntax to mention a username or team. How can such mentions be processed with Flexmark-Java? With following code snippet, Hello, @world ! is reported as a single Text node where I would expect to get world reported separately as some kind of MentionsNode:

final Parser parser = Parser.builder(new Parser.Builder()).build();
final Document document = parser.parse("Hello, @world !");
new NodeVisitor(Collections.emptyList()) {
       public void visit(Node node) {
           System.out.println("Node: " + node);
           super.visit(node);
       }
   }.visit(document);

Output:

Node: Document{}
Node: Paragraph{}
Node: Text{text=Hello, @world !}

There is a corresponding extension for this:

final MutableDataHolder options = new MutableDataSet()
                .set(Parser.EXTENSIONS, Collections.singletonList(GfmUsersExtension.create()));
final Parser parser = Parser.builder(options).build();
final Document document = parser.parse("Hello, @world, and #1!");
...