[BUGFIX] Allow editing of yaml files in the backend

This adds the yaml file extension to the known textfile extensions for
the backend, so that i.e. form manifests are editable in the backend.

Resolves: #80654
Releases: master
Change-Id: I9306211c16ec83128c002e04ba30dd7d6c1f80be
Reviewed-on: https://review.typo3.org/52334


Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Cedric Ziel <cedric@cedric-ziel.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>