[chore] complete and normalize the attributes of engine objects (#6258)

Drop outdated engine attributes: supported_languages, language_aliases

Complete, normalize and document the type definitions for the engine-module and
engine-class.

For the ``engine.about`` section of the configuration, a type check is performed
based on structure ``searx.enginelib.EngineAbout``.

The property ``engine.about.language`` no longer exists; existing values have
been migrated to ``engine.language``.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

docs/user/configured_engines.rst

@@ -58,8 +58,8 @@ Configured Engines
       {% for mod in engines %}
 
       * - `{{mod.name}} <{{mod.about and mod.about.website}}>`_
-          {%- if mod.about and  mod.about.language %}
-          ({{mod.about.language | upper}})
+          {%- if mod.language %}
+          ({{mod.language | upper}})
           {%- endif %}
         - ``!{{mod.shortcut}}``
         - {%- if 'searx.engines.' + mod.__name__ in documented_modules %}