[enh] theme/simple: custom router

Lay the foundation for loading scripts granularly depending on the endpoint it's
on.

Remove vendor specific prefixes as there are now managed by browserslist and
LightningCSS.

Enabled quite a few rules in Biome that don't come in recommended to better
catch issues and improve consistency.

Related:

- https://github.com/searxng/searxng/pull/5073#discussion_r2256037965
- https://github.com/searxng/searxng/pull/5073#discussion_r2256057100

client/simple/src/js/core/index.ts

@@ -0,0 +1,9 @@
+/**
+ * @preserve (C) Copyright Contributors to the SearXNG project.
+ * @preserve (C) Copyright Contributors to the searx project (2014 - 2021).
+ * @license AGPL-3.0-or-later
+ */
+
+import "./router.ts";
+import "./toolkit.ts";
+import "./listener.ts";