fix: use var instead of const for backward compatibility

Author: azu

lib/unist-util-map.js

@@ -1,6 +1,6 @@
 // LICENSE : MIT
 "use strict";
-const ObjectAssign = require("object-assign");
+var ObjectAssign = require("object-assign");
 module.exports = function map(ast, mapFn) {
     return (function preorder(node, index, parent) {
         const newNode = ObjectAssign({}, mapFn(node, index, parent));