WebUI: prefer arrow functions whenever applicable

Compared to plain function, arrow function is simpler to understand (without bindings to `this`, `arguments`, `super`) and to read.
Now, plain function will only be used when this object is required.

PR #21691.

src/webui/www/private/views/preferencesToolbar.html

@@ -32,7 +32,7 @@
 <script>
     "use strict";
 
-    (function() {
+    (() => {
         // Tabs
         MochaUI.initializeTabs("preferencesTabs");