[!!!][FEATURE] Separate internal_type=folder into an own type

The TCA type group became more and more a pure
database relation type throughout the years. The
only annoyance that was still there is internal_type
with "folder" set. Although they are very similar
in the user interface, internally they are handled
completely separate. They share only a fraction of
common TCA properties, too.

This leads to checks for internal_type !== 'folder'
everywhere across the codebase. To avoid this,
folder must become an own type.

With folder as an own type, the code becomes much
more readable and there is a lot less if-branching.

Resolves: #96983
Releases: main
Change-Id: Ifb746be9a5fc978a9a47af4c9e65bfc5498abf51
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73622


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>