[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/util/getElement.ts

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+import { assertElement } from "./assertElement.ts";
+
+type Options = {
+  assert?: boolean;
+};
+
+export function getElement<T>(id: string, options?: { assert: true }): T;
+export function getElement<T>(id: string, options?: { assert: false }): T | null;
+export function getElement<T>(id: string, options: Options = {}): T | null {
+  options.assert ??= true;
+
+  const element = document.getElementById(id) as T | null;
+
+  if (options.assert) {
+    assertElement(element);
+  }
+
+  return element;
+}