Convert auth() helper to use Auth facade

[barryvdh]

Summary

When the auth() helper is included, fix the return type to match \Auth

if (! function_exists('auth')) {
    /**
     * Get the available auth instance.
     *
     * @param  string|null  $guard
     * @return \Auth|($guard is null ? \Illuminate\Contracts\Auth\Factory : \Illuminate\Contracts\Auth\StatefulGuard)
     */
    function auth($guard = null)
    {
        if (is_null($guard)) {
            return app(AuthFactory::class);
        }

        return app(AuthFactory::class)->guard($guard);
    }
}

This will make auth()->user() behave the same as \Auth::user() which is returning the default User model already.

This might be useful to do for only the helpers that are actually required to change, like auth() and perhaps request()

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist

• Existing tests have been adapted and/or new tests have been added
• Update the README.md
• Code style has been fixed via composer fix-style