[TASK] npm script for grunt css build

Execute with "npm run build-css"

Change-Id: Ib5d57df2c6b1281baa245b36ba466524f8db65c0
Resolves: #81442
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/53059


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Jasmina Ließmann <code@frauliessmann.de>
Tested-by: Jasmina Ließmann <code@frauliessmann.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>