Make LocaleProvider return language instead of domain

[thomasdax98]

Demo: Part of PR: vivid-planet/comet#3451

Open questions

In Demo (TS v4) I typed scope explicitly:

<LocaleProvider resolveLocaleForScope={(scope: ContentScope) => scope.language}>

This doesn't work in the Starter (TS v5):

TS2322: Type (scope: ContentScope) => string is not assignable to type ResolveLocaleFunction
Types of parameters scope and scope are incompatible.
Type ContentScopeInterface is missing the following properties from type ContentScope: domain, language

Do we care? Any idea how I can make this work?

[johnnyomair]

We discussed this when we enabled TS Strict Mode: #314 (comment)

We'll probably need module augmentation for this.