[feature] elegant syntax update

[angauber]

This is an attempt at making this package more elegant / easy to use.

The default locale when using the model accessor is the app locale,
In some cases you might need to get / set translations with a custom locale and if you do so the accessor is not usable without overriding the app locale.

In most cases you could just override the app locale for a short period of time but this feels dirty.

Instead of using the app locale this PR adds a $locale property on the trait that is used instead of the app locale when set.

That makes it possible to use the accessor with any locale.

Example:
Imagine you want to save some data with the orm through a method:

Model:

// Model
class Item extends Model
{
    use HasTranslations;

    protected $fillable = ['name', 'info', 'state', 'comment'];

    public $translatable = ['name', 'info'];
}

Old way of doing it:

public function createTranslated($data, $locale)
{
    $test = new Test();

    $test->fill(Arr:except($data, $this->translatable));

    $test->setTranslations($locale, Arr::only($this->translatable))

    $test->save()
}

Or

    public function createTranslated($data, $locale)
    {
        $item = new Item();

        $buff = App::getLocale();
        App::setLocale($locale);

        $item->fill($data);

        $item->save();

        App::setLocale($buff);
    }

New way:

    public function createTranslated($data, $locale)
    {
        Item::withLocale($locale)->create($data);
    }

This is just a basic example that shows what issue this PR tries to overcome

[freekmurze]

Thanks!