[FEATURE] Load additional CSS files via TYPO3_CONF_VARS

This change deprecates the global array $TBE_STYLES
and with that the registration of CSS files for backend
styling, which is now handled via
$GLOBALS['TYPO3_CONF_VARS']['BE']['stylesheets']

Resolves: #100232
Releases: main
Change-Id: I36251d492563442fc82b43ec81a581f504583401
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78189


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Torben Hansen <derhansen@gmail.com>
Tested-by: Torben Hansen <derhansen@gmail.com>