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/scripts/rename-files.js

@@ -44,10 +44,10 @@ window.qBittorrent.MultiRename ??= (() => {
         replaceAll: false,
         fileEnumerationStart: 0,
 
-        onChanged: function(rows) {},
-        onInvalidRegex: function(err) {},
-        onRenamed: function(rows) {},
-        onRenameError: function(err) {},
+        onChanged: (rows) => {},
+        onInvalidRegex: (err) => {},
+        onRenamed: (rows) => {},
+        onRenameError: (err) => {},
 
         _inner_update: function() {
             const findMatches = (regex, str) => {