message: Prepend new "externals" options when merging webpack configs.
diff: @@ -58,6 +58,8 @@ const mergeWebpackConfigs = merge({
  });
  return [..._.unionWith(a, preAndPostRules, uniteRules.bind(null, {}, key)), oneOfRule];
  + } else if (key === "externals") {
  + return [...b, ...a];
  }
  // Fall back to default merging
  