[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>

searx/engines/demo_online.py

@@ -17,6 +17,7 @@ list in ``settings.yml``:
 
 from json import loads
 from urllib.parse import urlencode
+from searx.result_types import EngineResults
 
 engine_type = 'online'
 send_accept_language_header = True
@@ -70,21 +71,28 @@ def request(query, params):
     return params
 
 
-def response(resp):
+def response(resp) -> EngineResults:
     """Parse out the result items from the response.  In this example we parse the
     response from `api.artic.edu <https://artic.edu>`__ and filter out all
     images.
 
     """
-    results = []
+    res = EngineResults()
     json_data = loads(resp.text)
 
+    res.add(
+        res.types.Answer(
+            answer="this is a dummy answer ..",
+            url="https://example.org",
+        )
+    )
+
     for result in json_data['data']:
 
         if not result['image_id']:
             continue
 
-        results.append(
+        res.append(
             {
                 'url': 'https://artic.edu/artworks/%(id)s' % result,
                 'title': result['title'] + " (%(date_display)s) // %(artist_display)s" % result,
@@ -95,4 +103,4 @@ def response(resp):
             }
         )
 
-    return results
+    return res