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/package.json

@@ -13,6 +13,7 @@
     "@stylistic/eslint-plugin": "*",
     "eslint": "*",
     "eslint-plugin-html": "*",
+    "eslint-plugin-prefer-arrow-functions": "*",
     "eslint-plugin-regexp": "*",
     "html-validate": "*",
     "js-beautify": "*",