docs(chart-playground-web): add changelog entry for editor restore

Author: yordan-st

packages/pluggableWidgets/chart-playground-web/CHANGELOG.md

@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ## [Unreleased]
 
+### Changed
+
+- We restored the code editor in the playground with syntax highlighting and invalid-JSON feedback.
+
 ## [2.1.1] - 2025-07-15
 
 ### Changed