Added GDScript (#4315)

* Added GDScript

* Update free-programming-playgrounds.md

free-programming-playgrounds.md

@@ -8,6 +8,7 @@
 * [Docker](#docker)
 * [Elm](#elm)
 * [Flutter](#flutter)
+* [GDScript](#gdscript)
 * [Go](#go)
 * [Haskell](#haskell)
 * [Ionic](#ionic)
@@ -80,6 +81,11 @@
 * [Codepen](https://codepen.io/topic/flutter/templates)
 
 
+### GDScript
+
+* [GDScript](https://gdscript-online.github.io)
+
+
 ### Go
 
 * [Go Playground](https://play.golang.org)