Support locale for date formating

Consider this VTL expression:

```vtl
cast(cast("2000-" || MOIS_COLLECTE || "-01", date), string, "MMMM")
```

It currently always returns `October` but we would like to return the localized name instead (`Octobre` for a french locale).

Since we are using `date-fns` for that, it way be as simple as:

- get the browser locale (`navigator.language` ?),
- infer what to use for `format` [here](https://github.com/InseeFr/Trevas-JS/blob/master/packages/trevas/src/utils/dates.js#L11), by using the [proper `date-fns` option](https://date-fns.org/docs/format#).