[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/xpath.py

@@ -74,6 +74,7 @@ from urllib.parse import urlencode
 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
 
 search_url = None
 """
@@ -261,15 +262,15 @@ def request(query, params):
     return params
 
 
-def response(resp):  # pylint: disable=too-many-branches
-    '''Scrap *results* from the response (see :ref:`result types`).'''
+def response(resp) -> EngineResults:  # pylint: disable=too-many-branches
+    """Scrap *results* from the response (see :ref:`result types`)."""
+    results = EngineResults()
+
     if no_result_for_http_status and resp.status_code in no_result_for_http_status:
-        return []
+        return results
 
     raise_for_httperror(resp)
 
-    results = []
-
     if not resp.text:
         return results