Fix import of `$locales` from Typescript project

[allezxandre]

A top-level export in index.d.ts was preventing Typescript from using typings for $locales when imported from a Typescript project.

This PR fixes this by putting the export in a module declaration.

[allezxandre]

On a side note, Typescript will still complain about imports of actual locales in the form of import en from '$locales/en' as the $locales/* module is not defined.

[cibernox]

@allezxandre maybe there's a way of letting typescript know that $locales/<code> where code is any language code of the list of know language codes is valid.

In any case, this is a great improvement.

[allezxandre]

@allezxandre maybe there's a way of letting typescript know that $locales/<code> where code is any language code of the list of know language codes is valid.

Yes, setting something like so:

interface Locale {
    [key: string]: string;
}

declare module "$locales/*" {
    const locale: Locale;
    export default locale;
}

would work, but it would accept any language, including imports of random string like $locales/123R4ND0M.

There might be a better solution but alas I'm not experienced enough in Typescript module declarations to know 😕

[cibernox]

I don't see it very prioritary because most often than not apps should be using registerAll, unless they have a good reason not to (and since languages can be lazy loaded, there are very few good reasons)