src/document/doc-builders.js/ifBreak
function ifBreak(breakContents, flatContents, opts = {}) {
  if (process.env.NODE_ENV !== "production") {
    if (breakContents) {
      assertDoc(breakContents);
    }
    if (flatContents) {
      assertDoc(flatContents);
    }
  }

  return {
    type: "if-break",
    breakContents,
    flatContents,
    groupId: opts.groupId,
  };
}