[FEATURE] Add getEnv value modifier to TS parser

Use :=getEnv() in TypoScript to read values from environment variables.

Resolves: #85146
Releases: master
Change-Id: I6c480627e4b07f76f4d8887df0db9d95fd75e6d9
Reviewed-on: https://review.typo3.org/57115


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Hannes Lau <office@hanneslau.de>
Tested-by: Hannes Lau <office@hanneslau.de>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>