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Drop outdated engine attributes: supported_languages, language_aliases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

@@ -28,7 +28,7 @@ search_string = 'api/?{query}&limit={limit}'
 result_base_url = 'https://openstreetmap.org/{osm_type}/{osm_id}'
 
 # list of supported languages
-supported_languages = ['de', 'en', 'fr', 'it']
+photon_supported_languages = ["de", "en", "fr", "it"]
 
 
 # do search-request
@@ -37,7 +37,7 @@ def request(query, params):
 
     if params['language'] != 'all':
         language = params['language'].split('_')[0]
-        if language in supported_languages:
+        if language in photon_supported_languages:
             params['url'] = params['url'] + "&lang=" + language
 
     # using SearXNG User-Agent