FEATURE: Use diffhtml to update composer preview (#11237)

Displaying videos, animated GIFs or any kind of rich content in preview
used to refresh on every keystroke, which could cause performance
problems.

lib/tasks/javascript.rake

@@ -76,6 +76,9 @@ def dependencies
     }, {
       source: 'chartjs-plugin-datalabels/dist/chartjs-plugin-datalabels.min.js',
       public: true
+    }, {
+      source: 'diffhtml/dist/diffhtml.min.js',
+      public: true
     }, {
       source: 'magnific-popup/dist/jquery.magnific-popup.min.js',
       public: true