[TASK] Add css and less files to .editorconfig

Resolves: #63003
Releases: master, 6.2
Change-Id: I80a81e15afc71398d9607848160b8805407172fe
Reviewed-on: http://review.typo3.org/34236
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>