[!!!][FEATURE] Implement TypoScript function modifier event

The old TypoScript parser allows implementing own functions
for the := operator using a hook:

  someIdentifier := myCustomFunction(myFunctionArgument)

The new TypoScript parser does not implement this hook
and adds a new event instead.

Resolves: #98016
Related: #97816
Releases: main
Change-Id: I66dd4e393333c239b75a5935aacdbde46b1518e0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75280


Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>