Async-ified writing the tslint.json file in --init

[JoshuaKGoldberg]

The runWorker method is already async. Added a general purpose async wrapper in utils around fs.writeFile.

PR checklist

• Addresses an existing issue: Run linting logic in spawned threads #2328 (indirectly) / Convert fs.*Sync to async equivalents internally #3333

Overview of change:

Uses fs.readFile within an async wrapper under utils instead of fs.readFileSync.

[JoshuaKGoldberg]

GitHub isn't seeing an updated commit for the PR. Very annoying. Will re-create PR...