Feat/configurable fallback

[jeppester]

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Makes it possible to extend I18nMessagesProvider and override how fall back messages are generated.
For instance it will be easy to use fallback provided by i18n, so that a "translation missing" message will be displayed.

This is a draft, made for collecting feedback, and not to be merged in just yet.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[jeppester]

Another way to solve this could be to just have the following as a default:

if (i18n.locale === 'en') {
    return string.interpolate(defaultMessage, {
      field: fieldName,
      ...meta,
    })
}
return i18n.t(ruleIdentifier, meta)

Less flexible, but probably what most people would want 🤔