Add "ion.battery()" for obtain the current voltage of the battery

Ion.battery() return a float with the current battery voltage

TODO : Update the translation of the toolbox (only FR and EN currently)

python/port/mod/os/modos.cpp

@@ -111,5 +111,4 @@ mp_obj_t modos_listdir(void) {
   }
   
   return list;
-}
-
+}