src/language-markdown/printer-markdown.js/normalizeParts
function normalizeParts(parts) {
  return parts.reduce((current, part) => {
    const lastPart = privateUtil.getLast(current);

    if (typeof lastPart === "string" && typeof part === "string") {
      current.splice(-1, 1, lastPart + part);
    } else {
      current.push(part);
    }

    return current;
  }, []);
}