Allows the user to import a default aws_alb_listener_rule. When creating the resource with TF, the AWS API requires that `priority` be an integer.
However, the `DescribeRules` API call returns a string for `priority`. This would work in every case except for the `default` listener rule, which sets the `priority` value to the string "default".
This fixes the previous error:
```
Error importing: 1 error(s) occurred:
* aws_alb_listener_rule.test: Cannot convert rule priority %q to int: strconv.ParseInt: parsing "default": invalid syntax
```
Fixes: #11464