[BUGFIX] Make backend.user conditions in FE work

The TypoScript conditions "[backend.user.*]" are documented,
but were never implemented.

https://docs.typo3.org/m/typo3/reference-typoscript/master/en-us/Conditions/Index.html#backend-user

This patch is now introducing the missing conditions.

Resolves: #90767
Releases: master, 9.5
Change-Id: If6db277f56a2ef3457deb027a676af62a3494151
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63752


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Julian Mair <julian.mair94@gmail.com>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benni Mack <benni@typo3.org>