Complex numbers: new practice exercise

[colinleach]

These PRs are stacking up, so I'm happy to leave this auto-closed for now. It can be dealt with over the next few weeks/months.

Why "difficulty": 5? Python chose 4, F# 6, so splitting the difference seemed a reasonable starting point. It's not a decision I would defend to the death! Julia, for some reason, chose 10 - a bit over-generous.

[github-actions]

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[colinleach]

I was about to add this as a "new" practice exercise, then noticed that I submitted it nearly 2 years ago. I'll reopen for now, but mark it Draft until I can review the submission myself and see if I can improve it.

Update: configlet sync updated the instructions to match problem-specifications, and configlet fmt tidied up the config.json. I think it's time for a review.

[colinleach]

Please ignore most of the recent activity, I'm making a mess of things by pushing something to the wrong repo. I think I've reverted everything to yesterday evening's position.