Add Rego Playground to Playgrounds (EbookFoundation#12063)

* add Rego Playground

* Fix 2 of 2 Linter issues

* Resolved 1 of 1 review comment

Author: msaf9

more/free-programming-playgrounds.md

@@ -39,6 +39,7 @@
 * [R](#r)
 * [React](#react)
 * [Redis](#redis)
+* [Rego](#rego)
 * [Regular Expressions](#regular-expressions)
 * [Ruby](#ruby)
 * [Rust](#rust)
@@ -365,6 +366,11 @@
 * [Try Redis](https://try.redis.io)
 
 
+### Rego
+
+* [Rego Playground](https://play.openpolicyagent.org) - Open Policy Agent
+
+
 ### Regular Expressions
 
 * [iHateRegex: regex for playground](https://ihateregex.io/playground) - Geon George