Add Scastie (EbookFoundation#2702)

* Add Scastie

* Fix 'Remove trailing slash' warning

Author: alexanderfefelov

free-programming-playgrounds.md

@@ -120,5 +120,5 @@
 
 * [ScalaFiddle.io](https://scalafiddle.io)
 * [ScalaFiddle.net](http://scalafiddle.net)
-
+* [Scastie](https://scastie.scala-lang.org)