fix: convert copy-solutions script to .mjs

[JoshuaKGoldberg]

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PR Checklist

• [x] Addresses an existing open issue: fixes #264
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

[Example CI run] There are sed: can't read : No such file or directory logs showing up in CI that don't show up on my local Macs. The joy of shell scripts being different across operating systems!

Run ./scripts/copy-solutions.sh
sed: can't read : No such file or directory
sed: can't read : No such file or directory
Copying from "projects/classes/classifying-creatures/0[1](https://github.com/LearningTypeScript/projects/actions/runs/4747677348/jobs/8432892129#step:6:1)-polly-parrots-properties/solution.ts" to "projects/classes/classifying-creatures/01-polly-parrots-properties/index.ts".

This moves the script to all JavaScript so it's much more the same across operating systems.