Add 3 easy exercises

[ageron]

This PR adds 3 easy exercises:

• bob
• darts
• difference-of-squares

[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!

[ageron]

Note that I had to comment out three test cases in bob-tests.roc because the strings are not escaped properly for Roc by tojson. I'll fix this asap.

[Anton-4]

Thanks @ageron, looks good!