[TASK] Migrate SplitButtons to TypeScript

The SplitButtons module is migrated to TypeScript. Additionally, some
parameters of the Icons module are declared as optional.

Change-Id: I61f883e8e496e018a45f63c303de66274d071d94
Resolves: #82602
Releases: master
Reviewed-on: https://review.typo3.org/55906


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>