[TASK] Deprecate TYPO3_CONF_VARS_extensionAdded

The global array $GLOBALS['TYPO3_CONF_VARS_extensionAdded'] is widely
unknown and of little use.
Deprecate it along with helper method
ExtensionManagementUtility::appendToTypoConfVars()

Change-Id: Ia21324bece77aefee05fc3ff96ad756c5a0f0a12
Resolves: #80583
Releases: master
Reviewed-on: https://review.typo3.org/52271


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>