Add JS playground in zh_cn (#7596)

* Add JS playground in zh_cn

* Format removed slash

* Update more/free-programming-playgrounds-zh.md

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

more/free-programming-playgrounds-zh.md

@@ -1,8 +1,14 @@
 ### Index
 
 * [Dart](#dart)
+* [JavaScript](#javascript)
 
 
 ### Dart
 
 * [DartPad](https://dartpad.cn)
+
+
+### JavaScript
+
+* [码上掘金](https://code.juejin.cn) - 北京北比信息技术有限公司