[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Drop outdated engine attributes: supported_languages, language_aliases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/search/processors/__init__.py

@@ -11,6 +11,7 @@ __all__ = [
     "PROCESSORS",
     "ParamTypes",
     "RequestParams",
+    "ProcessorType",
 ]
 
 import typing as t
@@ -27,6 +28,14 @@ from .online_url_search import OnlineUrlSearchProcessor, OnlineUrlSearchParams
 
 logger = logger.getChild("search.processors")
 
+ProcessorType = t.Literal[
+    "offline",
+    "online",
+    "online_currency",
+    "online_dictionary",
+    "online_url_search",
+]
+
 OnlineParamTypes: t.TypeAlias = OnlineParams | OnlineDictParams | OnlineCurrenciesParams | OnlineUrlSearchParams
 OfflineParamTypes: t.TypeAlias = RequestParams
 ParamTypes: t.TypeAlias = OfflineParamTypes | OnlineParamTypes