Two fer

[TheRealOwenRees]

Add two-fer practice exercise.

This is a good place to test the test generator. All working fine, but it does highlight the need for copying exercise relevant files from a template directory on exercise creation.

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[TheRealOwenRees]

I believe that this is ready to merge. If you could give it a quick look before merging.

Before submitting any more exercises, I will get the template directory working with a command to sync all relevant files from there with the practice exercises