Allow other files than text-resources in config/texts folder

[nkylstad]

Description of the bug

All file names in the App/config/texts folder are used to fetch text resources for an app in Studio. However, if someone adds a custom code file or similar, the page displays an error message because files that are not text resources are also something our code attempts to fetch as if they were text resources.

We should consider adding a filter/regex when we read filenames from the texts folder, so that we only attempt to fetch the relevant files.

Steps To Reproduce

1. Go to an app in repos, and navigate to the App/config/texts folder
2. Add a new file and name it f.ex. testing.js, and commit
3. Go to the app in designer and fetch changes
4. Go to the texts page and see that an error message is shown
5. See in the network log that the languages endpoint returns js as one of the options
6. See in the network log that fetching resource.js.json fails

Additional Information

It is the languages endpoint that returns a list that is wrong.

[JamalAlabdullah]

Tested ok