Apply copyright check only on ".ts" files

Summary

I've noticed that the tool is complaining that copyright line in JSON files is not found.

Steps to reproduce

npm i (or npm run check-configuration) in @openstapps/backend with @openstapps/configuration: "0.18.0" (latest)

What is the current bug behavior?

The tool is complaining that JSON files (which are not supposed to and cannot have copyright header) don't have copyright line.

What is the expected correct behavior?

The tool should check only certain file types (.ts files).

Relevant logs and/or screenshots

Copyright line for file '/home/jkrunic/Desktop/OpenStApps/backend/src/storage/elasticsearch/templates/address.field.template.json' could not be found!

Copyright line for file '/home/jkrunic/Desktop/OpenStApps/backend/src/storage/elasticsearch/templates/article.sc-type.template.json' could not be found!

Copyright line for file '/home/jkrunic/Desktop/OpenStApps/backend/src/storage/elasticsearch/templates/base.template.json' could not be found!

Copyright line for file '/home/jkrunic/Desktop/OpenStApps/backend/src/storage/elasticsearch/templates/book.sc-type.template.json' could not be found!

Copyright line for file '/home/jkrunic/Desktop/OpenStApps/backend/src/storage/elasticsearch/templates/catalog.sc-type.template.json' could not be found!

...

Possible fixes

Only check .ts files.