[TASK] Add "ts" to the textfile_ext

The extension "ts" for typoscript is missing in the
default configuration. This patch adds it.

The goal is that the typoscript files can be edited
in the backend.

Resolves: #48812
Releases: 6.2
Change-Id: I40aca45f482e132b21aab03925d2cecd126e30ab
Reviewed-on: https://review.typo3.org/28317
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer