Export the locales list for isPostalCode

[stevula]

Ref #815 (resolves for postal code but not for phone)

Previously, the locales list was not exported at the top level. There was not a reliable way to check programmatically whether a country was supported before validating, which could result in errors.

This removes the need for such workarounds as:
import { locales } from 'validator/lib/isPostalCode';

This now works:
import { isPostalCodeLocales } from 'validator';

I used "isPostalCodeLocales" because an internal reference already existed to that name in the test file.

[chriso]

Thanks for the PR 😄