[refactor] typification of SearXNG / EngineResults

In [1] and [2] we discussed the need of a Result.results property and how we can
avoid unclear code.  This patch implements a class for the reslut-lists of
engines::

    searx.result_types.EngineResults

A simple example for the usage in engine development::

    from searx.result_types import EngineResults
    ...
    def response(resp) -> EngineResults:
        res = EngineResults()
        ...
        res.add( res.types.Answer(answer="lorem ipsum ..", url="https://example.org") )
        ...
        return res

[1] https://github.com/searxng/searxng/pull/4183#pullrequestreview-257400034
[2] https://github.com/searxng/searxng/pull/4183#issuecomment-2614301580
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

tests/unit/test_plugins.py

@@ -101,6 +101,6 @@ class PluginStorage(SearxTestCase):
             ret = self.storage.on_result(
                 sxng_request,
                 get_search_mock("lorem ipsum", user_plugins=["plg001", "plg002"]),
-                Result(results=[]),
+                Result(),
             )
             self.assertFalse(ret)