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/file-tree.js

@@ -88,7 +88,7 @@ window.qBittorrent.FileTree ??= (() => {
                 : this.nodeMap[rowId];
         },
 
-        getRowId: function(node) {
+        getRowId: (node) => {
             return node.rowId;
         },