fix: used a zx script for running changed solutions

[JoshuaKGoldberg]

PR Checklist

• [x] Addresses an existing open issue: fixes #257
• [x] That issue was marked as accepting prs
• [x] Steps in CONTRIBUTING.md were taken

Overview

Uses zx to write a JavaScript script that:

1. Collects all the unique changed projects
2. Runs the specified command in each of them
3. Complains if any failed

In theory this is all doable in bash. In practice I'm much better at JS/TS. 😄

Doesn't convert other scripts/*.sh files to a JS script. To my knowledge they're working file. This same conversion could be done to them later if needed.

[JoshuaKGoldberg]

FYI @fuadop since you'd put time into the old file. It's a good one - thank you again! 💖