[TASK] Mark various TypoScript parsing functionality as internal

The TypoScript Parser has been around since PHP 4 and hasn't
been touched much since 12ys. In order to speed up TypoScript
processing in the future, it is important to first streamline
all usages and mark internal functionality as "protected" instead
of "public".

Some other parts for sys_template rendering has been
marked as internal as well.

Resolves: #86461
Releases: master
Change-Id: I4ab0852bc0fb386aa9016b8845f7f8f15a7c3166
Reviewed-on: https://review.typo3.org/58477


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>