[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

searx/webapp.py

@@ -368,10 +368,10 @@ def get_pretty_url(parsed_url: urllib.parse.ParseResult):
 def get_client_settings():
     req_pref = sxng_request.preferences
     return {
+        'plugins': req_pref.plugins.get_enabled(),
         'autocomplete': req_pref.get_value('autocomplete'),
         'autocomplete_min': get_setting('search.autocomplete_min'),
         'method': req_pref.get_value('method'),
-        'infinite_scroll': req_pref.get_value('infinite_scroll'),
         'translations': get_translations(),
         'search_on_category_select': req_pref.get_value('search_on_category_select'),
         'hotkeys': req_pref.get_value('hotkeys'),