src/language-markdown/unified-plugins/front-matter.js/frontMatter
function frontMatter() {
  const proto = this.Parser.prototype;
  proto.blockMethods = ["frontMatter", ...proto.blockMethods];
  proto.blockTokenizers.frontMatter = tokenizer;

  function tokenizer(eat, value) {
    const parsed = parseFrontMatter(value);

    if (parsed.frontMatter) {
      return eat(parsed.frontMatter.raw)(parsed.frontMatter);
    }
  }
  tokenizer.onlyAtStart = true;
}