feat: add GetNationalPrefix method

[lawlielt]

Now that we are able to obtain the area code, but there is no method available to retrieve the national prefix. When displaying the area code and number separately to users, we hope to add the national prefix to the area code to facilitate users in obtaining customary telephone information.

[rowanseymour]

What's equivalent in https://github.com/google/libphonenumber ? We're trying to keep this a port of that library.

[lawlielt]

What's equivalent in https://github.com/google/libphonenumber ? We're trying to keep this a port of that library.

the GetNddPrefixForRegion method can do the same. :)