[FEATURE] Add "if" function to includeCSS and includeJS

[FEATURE] Add "if" function to includeCSS and includeJS

The includeJS / CSS / ... blocks lack the ability to
add TypoScript conditions to them. Even if some workarounds
exist, it doesn't seem to be too much overhead to add the
checks for a bit more convenience.

The related documentation changes are kept in issue #33724.

Change-Id: Ia4f6d8a176a7a7b613548f5481cced2e03df22b3
Fixes: #29423
Releases: 4.7
Reviewed-on: http://review.typo3.org/8844
Reviewed-by: Markus Klein
Reviewed-by: Alexander Grein
Tested-by: Alexander Grein
Reviewed-by: Henrik Ziegenhain
Reviewed-by: Stefan Galinski
Tested-by: Stefan Galinski