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Drop outdated engine attributes: supported_languages, language_aliases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

@@ -76,6 +76,9 @@ from lxml import html
 from searx.utils import extract_text, extract_url, eval_xpath, eval_xpath_list
 from searx.network import raise_for_httperror
 from searx.result_types import EngineResults
+from searx.enginelib import EngineAbout
+
+about = EngineAbout()
 
 search_url = None
 """