[enh] introduce Engine class

Previously engines were defined in modules, which where then
stapled with logger, supported_languages & language_aliases
via monkey patching in searx/engines/__init__.py.

Monkey patching is obviously a bad practice since it confuses
humans as well as static type checkers. But there were more problems:

* there were no type hints for the method input and output types

* the request method did not even make a clear distinction between input
  and output (abusing a single "params" dictionary for both)

This commit introduces a new class-based architecture for engines,
in a backwards-compatiable manner: allowing old-style module engines
to be used along witn new-style class-based engines.

docs/admin/engines/configured_engines.rst

@@ -48,12 +48,14 @@ Explanation of the :ref:`general engine configuration` shown in the table
 
       {% for mod in engines %}
 
+      {% set mod_name = mod['__name__'] or mod.__module__ %}
+
       * - `{{mod.name}} <{{mod.about and mod.about.website}}>`_
         - ``!{{mod.shortcut}}``
-        - {%- if 'searx.engines.' + mod.__name__ in documented_modules %}
-          :py:mod:`~searx.engines.{{mod.__name__}}`
+        - {%- if 'searx.engines.' + mod_name in documented_modules %}
+          :py:mod:`~searx.engines.{{mod_name}}`
           {%- else %}
-          :origin:`{{mod.__name__}} <searx/engines/{{mod.__name__}}.py>`
+          :origin:`{{mod_name}} <searx/engines/{{mod_name}}.py>`
           {%- endif %}
         - {{(mod.disabled and "y") or ""}}
           {%- if mod.about and  mod.about.language %}