Support passing array-like object in list formatting.

[myst729]

⚠️ This may be a big architectural change. However, I think it's worth doing for future extendability.

In need of propose a new feature (#214), I spent some time reading the source code. And found that list formatting is implemented individually with named formatting. For messages interpolated with "numbers":

messages: {
  en: {
    msg: 'This is {0} and {1}.'
  }
}

I could only pass an array:

<div>{{ $t('msg', ['bold', 'italic']) }}</div>

Passing an object (that look like an array) is not allowed:

<div>{{ $t('msg', {'0': 'bold', '1': 'italic'}) }}</div>

Following warning message appears, and the translation fails:

[vue-i18n] Type of token 'list' and format of value 'named' don't match!

This is a bit too strict to the users. And it doesn't behave consistently with JavaScript.

// These two expressions output the same result: ['0', '1']
Object.keys(['bold', 'italic'])
Object.keys({'0': 'bold', '1': 'italic'})

Most i18n libs or components like Dojo, Angular, etc are making list formatting syntax sugar of named formatting. Since in JavaScript, arrays are objects too.

This benefits both sides:

• To the users, it brings flexibility of formatting messages interpolated with "numbers".
• To the developers, reuse of such mechanism makes it cleaning under the hood. And easier to ship new features.

[myst729]

The change is actually much easier than I thought, and has been implemented in #218