AbstractFormElementContainer: Add method `hasElements()` #146

Job	Run time
PHPStan with PHP 7.4 prefer-stable	1m 9s
PHPStan with PHP 7.4 prefer-lowest	46s
PHPStan with PHP 8.0 prefer-stable	1m 9s
PHPStan with PHP 8.0 prefer-lowest	1m 8s
PHPStan with PHP 8.3 prefer-stable	2m 55s
PHPStan with PHP 8.3 prefer-lowest	2m 59s
	10m 6s

Provide feedback