[BUGFIX] Enable "show source" btn in default RTE config

The button was accidentally removed when introducing
the configuration of the RTE via YAML.

Resolves: #79807
Releases: master
Change-Id: I083432af50279f7bc3a1e0d0d748d8ee06b31354
Reviewed-on: https://review.typo3.org/51670


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Tested-by: Jigal van Hemert <jigal.van.hemert@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>