feat: export locale

[fjc0k]

This is useful for custom method. eg:

import * as yup from 'yup'
import { isChineseMobilePhoneNumber } from '../utils'

yup.setLocale({
  string: {
    chineseMobilePhoneNumber: '${path} must be a Chinese mobile phone number'
  }
})

yup.addMethod(
  yup.string,
  'chineseMobilePhoneNumber',
  function (message = yup.locale.string.chineseMobilePhoneNumber) {
    return this.test(
      'chineseMobilePhoneNumber',
      message,
      isChineseMobilePhoneNumber
    )
  }
)

[tpict]

You can import this from yup/lib/locale, but that causes issues for TypeScript users because the @types/yup declaration file won't be applied to that module. This PR would fix that issue!

[knicola]

we can now use defaultLocale ie. yup.defaultLocale.string though the internal locale types, such as StringLocale, are not exported so declaration merging is not working.