Add practice exercise split-second-stopwatch

[jiegillet]

Brand new exercise!

I have been using Cursor for a little while, and I must say that this really helps with implementing tests like that. You make one change in the test files, tab-tab-tab and all the next ones change as well. You gotta be careful, but it really saves time. It also pretty much implemented the solution without me really asking for it. Pretty interesting experience.

[github-actions]

Thank you for contributing to exercism/elixir 💜 🎉. This is an automated PR comment 🤖 for the maintainers of this repository that helps with the PR review process. You can safely ignore it and wait for a maintainer to review your changes.

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• Any exercise changed

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• Practice exercise changed

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[jiegillet]

Oh no! I forgot that Duration was only introduced in Elixir 1.17, we need to support from 1.13.
I'll think of some other way to implement it, maybe using Time, but not today, so I'll set this back to draft.

[jiegillet]

Done. Using Time actually made the code cleaner.

[neenjaw]

LGTM