[TASK] Extract displayCond functionality from FormEngine to new class

In order to streamline \TYPO3\CMS\Backend\Form\FormEngine
(formerly t3lib_TCEforms) the functionality for 'displayCond' is extracted
into a new class: \TYPO3\CMS\Backend\Form\ElementConditionMatcher

* Refactor the code a bit without changing it's behaviour
* Deprecate FormEngine->isDisplayCondition() and remove its core usages
* Add a bit of documentation in the new class
* Add Unit tests

Releases: 6.1
Resolves: #46461
Change-Id: Ie199fe0600a70231f1e7b710a09164364cef8578
Reviewed-on: https://review.typo3.org/19080
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn