[TASK] Restructure form setup

The setup of the "form" extension has been restructured. For each
validator, form element, and finisher separate files are available.

In addition, inheritances have been resolved, which makes existing
mixins obsolete. In a subsequent change these mixins will be deprecated
as these where actually meant for internal use only but could be used
by 3rd party extensions.

Due to this change, integrators can easily understand how form objects
are defined.

Resolves: #84221
Resolves: #84203
Releases: master
Change-Id: I1b8d0d67929015ed687335586875fde73b598c13
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62366


Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniela Pohl <dpohl@i-d.de>
Reviewed-by: Martin Schöne <martin.koske@gmx.de>
Reviewed-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniela Pohl <dpohl@i-d.de>
Tested-by: Martin Schöne <martin.koske@gmx.de>
Tested-by: Ralf Zimmermann <ralf.zimmermann@tritum.de>