Add an optional state/region/province text input if there are no entries in states.php for a country

**Is your feature request related to a problem? Please describe.**
Many countries do not have a listing of states/regions/provinces/etc in states.php, but can have one optionally entered.

**Describe the solution you'd like**
Instead of just hiding the state dropdown, switch it to an input with the description from states.php as the placeholder, with "(Optional)" appended. This is similar to what WooCommerce does currently.

**Describe alternatives you've considered**
Since it's not required we could just leave this out unless there are complaints.