src/language-markdown/printer-markdown.js/printLine
function printLine(path, value, options) {
  if (options.proseWrap === "preserve" && value === "\n") {
    return hardline;
  }

  const isBreakable =
    options.proseWrap === "always" &&
    !getAncestorNode(path, SINGLE_LINE_NODE_TYPES);
  return value !== ""
    ? isBreakable
      ? line
      : " "
    : isBreakable
    ? softline
    : "";
}