[BUGFIX] Ensure correct custom (sub)category handling in ConstantEditor

The constant editor has been revamped with #98357 using the
new TypoScript parser introduced with TYPO3 v12 under the
hood.

For category and subCategory handling lower-cased keys have
been used, missing to lower-case the custom categories and
subcategories names (keys) integrator are able to add since
aeons. That leads to a failing match later on, and adding
constants to the generic `other` subcategory.

The revamped code supports only one category per subCategory
sorting value. The documentation say that this **should** be
omitted, but it worked in the past and broke with the revamped
implementation.

This change fixes muliple issues with the constant editor:

* Ensure that custom categories and subcategories are read
  lower-case, otherwise later lookup will fail and sorted
  into sub-categorie `other`.
* Add addtional array level to have `subCategory->items`
  per second sorting value as array (to avoid overriding).
* Add additional item loop to the fluid template to respect
  the additional array level.

Note: In the past it was possible to display a constant
in multiple categories/subcategories by adding them again
with different category configure comment lines. This is
not supported anymore and not considered as bug.

Resolves: #103088
Related: #98357
Related: #97816
Releases: main, 12.4
Change-Id: I4403420cc957cdef4bf077365deac0f7af621c71
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82972


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>