[mod] client/simple: client plugins (#5406)

* [mod] client/simple: client plugins

Defines a new interface for client side *"plugins"* that coexist with server
side plugin system. Each plugin (e.g., `InfiniteScroll`) extends the base
`ts Plugin`. Client side plugins are independent and lazy‑loaded via `router.ts`
when their `load()` conditions are met. On each navigation request, all
applicable plugins are instanced.

Since these are client side plugins, we can only invoke them once DOM is fully
loaded. E.g. `Calculator` will not render a new `answer` block until fully
loaded and executed.

For some plugins, we might want to handle its availability in `settings.yml`
and toggle in UI, like we do for server side plugins. In that case, we extend
`py Plugin` instancing only the information and then checking client side if
[`settings.plugins`](https://github.com/inetol/searxng/blob/1ad832b1dc33f3f388da361ff2459b05dc86a164/client/simple/src/js/toolkit.ts#L134)
array has the plugin id.

* [mod] client/simple: rebuild static

client/simple/src/js/loader.ts

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import type { Plugin } from "./Plugin.ts";
+import { type EndpointsKeys, endpoint } from "./toolkit.ts";
+
+type Options =
+  | {
+      on: "global";
+    }
+  | {
+      on: "endpoint";
+      where: EndpointsKeys[];
+    };
+
+export const load = <T extends Plugin>(instance: () => Promise<T>, options: Options): void => {
+  if (!check(options)) return;
+
+  void instance();
+};
+
+const check = (options: Options): boolean => {
+  // biome-ignore lint/style/useDefaultSwitchClause: options is typed
+  switch (options.on) {
+    case "global": {
+      return true;
+    }
+    case "endpoint": {
+      if (!options.where.includes(endpoint)) {
+        // not on the expected endpoint
+        return false;
+      }
+
+      return true;
+    }
+  }
+};