[BUGFIX] Allow whitespaces in list of file extensions

This ensures that spaces can be used to format the list of file
extensions for <INCLUDE_TYPOSCRIPT>.

Resolves: #81709
Releases: master, 8.7
Change-Id: Ia0c830df52b18a3dae7bd724c4bd5ee3b845e00f
Reviewed-on: https://review.typo3.org/53337


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>