[TASK] Make CharsetConverter (csConv) a singleton

The CharsetConverter is agnostic to the surroundings,
so it can be simply be instantiated once with a Singleton
at any time.

Resolves: #72335
Releases: master
Change-Id: I1989480c56c2db8f015501ae9f82ae8f2384ea13
Reviewed-on: https://review.typo3.org/45377


Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>