Define an actual interface for IntLocaleData

[ranst91-u]

The IntLocaleData TS interface looks like this:
interface IntlLocaleData {}

My suggestion is to define an actual interface for this.
Otherwise, when trying to grab a property of a specific locale object, every property I'll try to get, will hit a Property '...' does not exist on type 'LocaleData'

Otherwise, there is no way to work with the default export of the locale data with TS

[jamuhl]

Feel free to provide a PR