🛠 Tooling: Use `.ts` extensions for relative import paths

### Bug Report Checklist - [x] I have tried restarting my IDE and the issue persists. - [x] I have pulled the latest `main` branch of the repository. - [x] I have [searched for related issues](https://github.com/JoshuaKGoldberg/sentences-per-line/issues?q=is%3Aissue) and found none that matched my issue. ### Overview Filing for #826: to make source files more directly map what they're actually doing (importing `.ts` files). ### Additional Info 📐