AK: Make StringBuilder::join() use appendff() instead of append()

`append()` is almost never going to select the overload that is desired.
e.g. it will append chars when you pass it a Vector<size_t>, which is
definitely not the right overload :)

AK/StringBuilder.h

@@ -57,7 +57,7 @@ public:
                 first = false;
             else
                 append(separator);
-            append(item);
+            appendff("{}", item);
         }
     }