Add getFallbackLocale method

[gdebrauwer]

In a project I'm working on, my models have translatable attributes but also a 'fallback_locale' attribute. If for example the title of the model does not have a translation in the user's current locale, then it should use the translation in the 'fallback_locale' that is set on the model (instead of a global fallback locale that is set for entire application). To support this behavior, this PR adds the option to define a getFallbackLocale method on a model

class Article extends Model
{
    use HasTranslations;

	protected $translatable = [
       'title',
    ];

    public function getFallbackLocale() : string
    {
        return $this->fallback_locale; // a column/attribute on the model
    }
}

$article->setTranslation('title', 'fr', 'french title');
$article->setTranslation('title', 'en', 'english title');
$article->forceFill(['fallback_locale' => 'fr']);

config()->set('app.fallback_locale', 'en');

app()->setLocale('nl');

// The french translation is returned because no dutch translation exist
dump($article->title);

[freekmurze]

Looks good!

Could you also add an example for this in the readme?

[gdebrauwer]

✅ done

[freekmurze]

Perfect, thanks!