reverse-string: Add generator and regenerate tests

[tasxatzial]

Also,

• Updated example solution
• Updated function template
• Excluded the two grapheme tests:
  • They would invalidate all solutions
  • They would invalidate all approaches
  • I wasn't able to come up with a simple solution that passed those tests.

[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.

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[no important files changed]

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