[FEATURE] Adds register values to FilesContentObj

Adds FILE_NUM_CURRENT and FILES_COUNT to the register
for each file.

Use case:
10 = FILES
10 {
	references {
		table = tt_news
		uid.field = uid
		fieldName = media
	}
	renderObj = COA
	renderObj {
		10 = TEXT
		10.value = Render first file two times
		10.if.isFalse.data = register:FILE_NUM_CURRENT
		20 = TEXT
		20.value = file {register:FILE_NUM_CURRENT} of {register:FILES_COUNT}
		20.insertData = 1
	}
}

Resolves: #49480
Releases: 6.2
Change-Id: I3eca404210ee21f73cea0860fa0be7cc39b4632f
Reviewed-on: https://review.typo3.org/21747
Reviewed-by: Lars Peipmann
Tested-by: Lars Peipmann
Tested-by: Wouter Wolters
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind