[feat] sourcehut engine: implement as custom module, fix user agent

SourceHut uses a foss bot protection tool called `go-away` (which I can
recommend BTW).  It blocks common crawler user agents, such as the standard
Firefox user agent.  Hence, we're now using our custom SearXNG user agent to
clarify we're not a crawler.

Closes: https://github.com/searxng/searxng/issues/5270
Co-authored-by: Markus Heiser <markus.heiser@darmarit.de>

searx/settings.yml

@@ -2677,23 +2677,10 @@ engines:
 
   - name: sourcehut
     shortcut: srht
-    engine: xpath
-    paging: true
-    search_url: https://sr.ht/projects?page={pageno}&search={query}
-    results_xpath: (//div[@class="event-list"])[1]/div[@class="event"]
-    url_xpath: ./h4/a[2]/@href
-    title_xpath: ./h4/a[2]
-    content_xpath: ./p
-    first_page_num: 1
-    categories: [it, repos]
+    engine: sourcehut
+    # https://docs.searxng.org/dev/engines/online/sourcehut.html
+    # sourcehut_sort_order: longest-active
     disabled: true
-    about:
-      website: https://sr.ht
-      wikidata_id: Q78514485
-      official_api_documentation: https://man.sr.ht/
-      use_official_api: false
-      require_api_key: false
-      results: HTML
 
   - name: goo
     shortcut: goo