feat: allow composable

[dargmuesli]

Uses https://github.com/intlify/vue-i18n-composable/ to allow usage of this module in Vue 2.7's setup function. The currently supported usage outside of the setup functions remains working in my testing.

[dargmuesli]

Could close #1333 as well.

[rchl]

According to its documentation, the v2 version only works with Vue 2.7 and not 2.6 so we can't use it.

[rchl]

And note that this would only cover the vue-i18n APIs while this module provides a bunch of additional API that are specific to this module.

[dargmuesli]

You're right, I knew this would be too easy 😄 So the only option is to wait for the next branch for composition support? Or are there any other possibilities to use the composition api with Vue 2.7 and vue-i18n v8 (= nuxt-i18n v7)?

[dargmuesli]

I made different branches for different extensions of nuxt-i18n, like with vue-i18n-composable or vue-i18n-bridge in https://github.com/dargmuesli/nuxt-i18n-module/. For anyone interested in trying one of those out while v8 and v7.3 of nuxt-i18n are in progress, feel free to have a look! Closing this PR in favor of @next and a Nuxt v3 migration 🤓.