[TASK] Cleanup form element reference in FormEngine module

The TypeScript part of the FormEngine got the form element in a weird
way all over again. To make the code a little bit cleaner, the form
element is now accessed using a `FormEngine` getter property
in order to ensure types are properly infered.

Resolves: #101836
Releases: main, 12.4
Change-Id: I737287fc9a0b8fd94cc3bf915e7f1d2511c720ac
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80880


Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>