[TASK] Add Fluid expressions and pre-processors global config

This patch moves a previously hardcoded list of Fluid
implementations from the RenderingContext constructor,
to TYPO3_CONF_VARS. Extensions and site administrators
can then add their own implementations to this array and
have them be used in every RenderingContext.

See included RST file for further information on usage.

Change-Id: I6190ceb410e1121ea935a31237cb8bcd0b2f0417
Resolves: #82763
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/54395


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mattias Nilsson <tollepjaer@gmail.com>
Tested-by: Mattias Nilsson <tollepjaer@gmail.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Jan Stockfisch <jan.stockfisch@googlemail.com>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>