Add prettier config file + recommended extension

WARNING: Really sorry, I tried everything I can but for some reasons, I cannot make a merge request with only the 2 files I commited !! I would be happy to learn how to do it properly ...

The only changes are the last 2 files .prettierrc and .vscode/extensions.json

lib/ui/widgets/first_screen/pay_form.dart

@@ -54,7 +54,7 @@ class _PayFormState extends State<PayForm> {
 
       final bool sentDisabled =
           _onPressed(state, context, currency, currentUd) == null;
-      final Color sentColor = !sentDisabled
+      final Color sentColor = sentDisabled
           ? Theme.of(context).disabledColor
           : isDark(context)
               ? const Color(0xFFB8D166)