[BUGFIX] Treat <pre> tags correctly in RTE

[BUGFIX] Treat <pre> tags correctly in RTE

Do not throw away line breaks and multiple spaces of pre tag's content in RTE.
This finally allows to paste source code and keep its formatting.

Resolves: #78186
Releases: master, 7.6
Change-Id: I183032d53bf64c8f1e3acb8222288860f97fed33
Reviewed-on: https://review.typo3.org/50113
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Martin Fox <kontakt@mlis.pl>
Tested-by: Martin Fox <kontakt@mlis.pl>
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>