style: upgrade JsHint to EcmaScript 2018 as ESLint to align them

.jshintrc

@@ -2,7 +2,7 @@
     "bitwise": true,
     "curly": true,
     "eqeqeq": true,
-    "esversion": 6,
+    "esversion": 2018,
     "forin": true,
     "freeze": true,
     "futurehostile": true,