[TASK] Use constants for checking CharsetConverter

The inner parts of CharsetConverter are checking for
the TYPO3_CONF_VARS setting. This is now cleaned up
by using a protected method return constants.

This is a pre-step towards migrating the current options.

Resolves: #72503
Releases: master
Change-Id: Id1ba74d012f87647846bc11419110d9d9ed23082
Reviewed-on: https://review.typo3.org/45547


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>