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Drop outdated engine attributes: supported_languages, language_aliases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

@@ -23,8 +23,8 @@ about = {
     "use_official_api": False,
     "require_api_key": False,
     "results": "JSON",
-    "language": "zh",
 }
+language = "zh"
 
 paging = True
 categories = []