[FEATURE] Allow .ts file extension for static typoscript templates

At the moment the following static typoscript filenames are allowed:

setup.txt
constants.txt
include_static.txt
include_static_files.txt

* Allow ".ts" as file extensions
* Allow mixed usage of .ts and .txt
* .ts precedes .txt

Change-Id: I0ffd9ef50a07dfbaa8388d525c5ced09d5070103
Fixes: #34922
Releases: 4.8
Reviewed-on: http://review.typo3.org/9736
Reviewed-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog