AK: Fix URL::complete_url behaviour for when a fragment is passed

Previously, passing a fragment string ("#section3") to the complete_url
method would result in a URL that looked like
"file:///home/anon/www/#section3" which was obviously incorrect. Now the
result looks like "file:///home/anon/www/afrag.html#section3".

AK/URL.cpp

@@ -307,6 +307,12 @@ URL URL::complete_url(const String& string) const
         return url;
     }
 
+    if (string.starts_with("#")) {
+        url = *this;
+        url.set_fragment(string.substring(1, string.length() - 1));
+        return url;
+    }
+
     StringBuilder builder;
     FileSystemPath fspath(path());
     builder.append('/');