[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

docs/dev/plugins/builtins.rst

@@ -10,6 +10,7 @@ Built-in Plugins
    calculator
    hash_plugin
    hostnames
+   infinite_scroll
    self_info
    tor_check
    unit_converter

docs/dev/plugins/infinite_scroll.rst

@@ -0,0 +1,8 @@
+.. _plugins.infinite_scroll:
+
+===============
+Infinite scroll
+===============
+
+.. automodule:: searx.plugins.infinite_scroll
+   :members: