add some packages

node_modules/core-js/modules/es7.string.at.js

@@ -0,0 +1,10 @@
+'use strict';
+// https://github.com/mathiasbynens/String.prototype.at
+var $export = require('./_export');
+var $at = require('./_string-at')(true);
+
+$export($export.P, 'String', {
+  at: function at(pos) {
+    return $at(this, pos);
+  }
+});