[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Drop outdated engine attributes: supported_languages, language_aliases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/engines/duden.py

@@ -14,8 +14,8 @@ about = {
     "use_official_api": False,
     "require_api_key": False,
     "results": 'HTML',
-    "language": 'de',
 }
+language = "de"
 
 categories = ['dictionaries']
 paging = True