feat: built-in fluent integration

[dcdunkan]

Makes the Fluent class taken from better_fluent built-in.

There are several reasons for this change:

• I'll make a PR for https://t.me/grammyjs/103520 next (I already have a implementation). For that we need a custom syntax for keys, and a custom way to manage the bundles. Its not something that I currently want to add to better_fluent. This PQ makes it easy to implement that feature.
• better_fluent can handle multiple resources for the same locale, which is not a feature being used in i18n. So, this fluent has less code, and makes it a little faster.
• Closes Fluent dependencies are bundled on Node instead of being added to dependencies #29 because only x/fluent is imported and that can be mapped to @fluent/bundle and @fluent/langneg in the dnt configuration. So, no transforming is required at all for the dependencies.
• Closes Narrow interfaces are not assignable to TranslationContext #28 as I've made the fixes and changes of Support interfaces as translation context #30 in this PQ inside the Fluent class as well.
• And something not so important: Don't have to depend on my repository, So, even when I'm unavailable others can make the needed changes that's useful for i18n (I was and I'll be inactive for few months)

I don't think this has breaking changes in the point-of-view of i18n, because only the #30 changes are added in to I18n.

Feedback welcome.

[rojvv]

LGTM.

[KnorpelSenf]

@dcdunkan or @EdJoPaTo feel free to release