[FEATURE] Hook for tsfe::checkEnableFields

This hook for tsfe::checkEnableFields allows extensions to provide
additional rules for page visibility when parent pages have
"extend to subpages" activated.

Change-Id: I6ef68802058863c9629333b59c0806dd2ab6c471
Resolves: #48981
Releases: 6.2
Reviewed-on: https://review.typo3.org/21306
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring