[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Drop outdated engine attributes: supported_languages, language_aliases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>

searx/templates/simple/preferences/engines.html

@@ -60,8 +60,8 @@
                 {%- endif -%}
                 <label for="{{ engine_id }}">
                   {{- ' ' -}}{{- search_engine.name -}}
-                  {%- if search_engine.about and search_engine.about.language -%}
-                    {{- ' ' -}}({{search_engine.about.language | upper}})
+                  {%- if search_engine.language -%}
+                    {{- ' ' -}}({{search_engine.language | upper}})
                   {%- endif -%}
                   </label>
                   {{- engine_about(search_engine) -}}