Refactor VueI18n creation to own module

[chrismayer]

This refactors the creation of the VueI18n instance by encapsulating it in an own ES module. So the i18n instance can be imported to other pure ES modules and will allow internationalization there.

This follows the approach given here: kazupon/vue-i18n#149 (comment)

This would make the introduction of a global $appLanguage variable in #356 obsolete by this much cleaner and more versatile approach. If you approve this PR, I'd happily revert #356 afterwards in a separate PR.

Accessing the current app language in pure ES modules could then be done by the following code:

const { i18n } = await import('../../../src/locales/wgu-i18n.js');
console.log(i18n.locale);

[sronveaux]

Works completely as expected. As said, this one renders #356 obsolete.

The only thing I wanted to comment on is that assignment of $appConfig on Vue.prototype directly inside unit tests is considered bad practice. We should create a local Vue instance and assign to this local instance in place of the global one.
However, as this is also done in other modules (for example LocaleSwitcher.spec.js), I approved as is. Perhaps an issue should be created with this point to be addressed and enhanced later...

[chrismayer]

Thanks for your review @sronveaux and thanks for your suggestion regarding the assignment to Vue.prototype in the tests. I created an issue to improve that as a whole ➡️ #388. Going to merge now...