[12.x] Add whenFails and whenPasses methods on Validator

[gdebrauwer]

This PR introduces whenFails and whenPasses methods on the Validator class so you can fluently react to the validation passing or failing.

This would be useful when you use the validator to validate data outside the request cycle, for example, data that is provided to a constructor or a method on a class.

// Before

public function someMethod($file)
{
	$validator = Validator::make(
		['file' => $file],
		['file' => 'required|image|dimensions:min_width=100,min_height=200']
	);
	
	if ($validator->fails()) {
		throw new InvalidArgumentException('Provided file is invalid');
	}
}

// After

public function someMethod($file)
{
	Validator::make(
		['file' => $file],
		['file' => 'required|image|dimensions:min_width=100,min_height=200']
	)->whenFails(function () {
		throw new InvalidArgumentException('Provided file is invalid');
	});
}