Rule::Unless validation rule

[msmahon]

The when/unless paradigm already exists in many places. It seems intuitive to include that paradigm in the Illuminate/Validation/Rule class.

Existing "When" method:

request()->validate([
    'credential' => [
        'min:5',
        Rule::when($isNotAdmin, ['required'])
    ],
]);

Proposed "Unless" method:

request()->validate([
    'credential' => [
        'min:5',
        Rule::unless($isAdmin, ['required'])
    ],
]);

As an addition to the Rule class:

    /**
     * Create a new inverted conditional rule set.
     *
     * @param  callable|bool  $condition
     * @param  array|string|\Closure  $rules
     * @param  array|string|\Closure  $defaultRules
     * @return \Illuminate\Validation\ConditionalRules
     */
    public static function unless($condition, $rules, $defaultRules = [])
    {
        return new ConditionalRules(! $condition, $rules, $defaultRules);
    }