[TASK] Use CodeMirror for backend layout code preview

Currently the preview of the generated code for
a backend layout uses only <code> tag.
To streamline the ui this will use the t3editor
if installed to display the generated code

Resolves: #94222
Releases: master
Change-Id: I1df1d03af120c50662ec5806c067b9af49c85dc5
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69308


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>