[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/Plugin.ts

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+/**
+ * Base class for client-side plugins.
+ *
+ * @remarks
+ * Handle conditional loading of the plugin in:
+ *
+ *   - client/simple/src/js/router.ts
+ *
+ * @abstract
+ */
+export abstract class Plugin {
+  /**
+   * Plugin name.
+   */
+  protected readonly id: string;
+
+  /**
+   * @remarks
+   * Don't hold references of this instance outside the class.
+   */
+  protected constructor(id: string) {
+    this.id = id;
+
+    void this.invoke();
+  }
+
+  private async invoke(): Promise<void> {
+    try {
+      console.debug(`[PLUGIN] ${this.id}: Running...`);
+      const result = await this.run();
+      if (!result) return;
+
+      console.debug(`[PLUGIN] ${this.id}: Running post-exec...`);
+      // @ts-expect-error
+      void (await this.post(result as NonNullable<Awaited<ReturnType<this["run"]>>>));
+    } catch (error) {
+      console.error(`[PLUGIN] ${this.id}:`, error);
+    } finally {
+      console.debug(`[PLUGIN] ${this.id}: Done.`);
+    }
+  }
+
+  /**
+   * Plugin goes here.
+   *
+   * @remarks
+   * The plugin is already loaded at this point. If you wish to execute
+   * conditions to exit early, consider moving the logic to:
+   *
+   *   - client/simple/src/js/router.ts
+   *
+   * ...to avoid unnecessarily loading this plugin on the client.
+   */
+  protected abstract run(): Promise<unknown>;
+
+  /**
+   * Post-execution hook.
+   *
+   * @remarks
+   * The hook is only executed if `#run()` returns a truthy value.
+   */
+  // @ts-expect-error
+  protected abstract post(result: NonNullable<Awaited<ReturnType<this["run"]>>>): Promise<void>;
+}