[mod] typification of SearXNG: add new result type File

This PR adds a new result type: File

    Python class: searx/result_types/file.py
    Jinja template: searx/templates/simple/result_templates/file.html
    CSS (less) client/simple/src/less/result_types/file.less

Class 'File' (singular) replaces template 'files.html' (plural).  The renaming
was carried out because there is only one file (singular) in a result. Not to be
confused with the category 'files' where in multiple results can exist.

As mentioned in issue [1], the class '.category-files' was removed from the CSS
and the stylesheet was adopted in result_types/file.less (there based on the
templates and no longer based on the category).

[1] https://github.com/searxng/searxng/issues/5198

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings.yml

@@ -2298,31 +2298,27 @@ engines:
 
   - name: wikicommons.images
     engine: wikicommons
-    shortcut: wc
+    shortcut: wci
     categories: images
-    search_type: images
-    number_of_results: 10
+    wc_search_type: image
 
   - name: wikicommons.videos
     engine: wikicommons
     shortcut: wcv
     categories: videos
-    search_type: videos
-    number_of_results: 10
+    wc_search_type: video
 
   - name: wikicommons.audio
     engine: wikicommons
     shortcut: wca
     categories: music
-    search_type: audio
-    number_of_results: 10
+    wc_search_type: audio
 
   - name: wikicommons.files
     engine: wikicommons
     shortcut: wcf
     categories: files
-    search_type: files
-    number_of_results: 10
+    wc_search_type: file
 
   - name: wolframalpha
     shortcut: wa