[TASK] Update less class state in TypoScript tokenizers

Detail methods of the two TypoScript tokenizers tend
to add mental load since they often update `$this`
state.

The patch changes `splitLines()` and `parseValueForConstants()`
to return updated things instead, for consuming methods
to take care of this.

This is a pre-patch to simplify #102742.

Resolves: #103944
Related: #102742
Releases: main, 12.4
Change-Id: I1a5dbccc1bdfbfc52681e323eb1fe9a2b0bb7dc7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84439


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>