[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/router.ts

@@ -0,0 +1,38 @@
+import { Endpoints, endpoint, ready, settings } from "./toolkit.ts";
+
+ready(
+  () => {
+    import("../main/keyboard.ts");
+    import("../main/search.ts");
+
+    if (settings.autocomplete) {
+      import("../main/autocomplete.ts");
+    }
+  },
+  { on: [endpoint === Endpoints.index] }
+);
+
+ready(
+  () => {
+    import("../main/keyboard.ts");
+    import("../main/mapresult.ts");
+    import("../main/results.ts");
+    import("../main/search.ts");
+
+    if (settings.infinite_scroll) {
+      import("../main/infinite_scroll.ts");
+    }
+
+    if (settings.autocomplete) {
+      import("../main/autocomplete.ts");
+    }
+  },
+  { on: [endpoint === Endpoints.results] }
+);
+
+ready(
+  () => {
+    import("../main/preferences.ts");
+  },
+  { on: [endpoint === Endpoints.preferences] }
+);