Refactor concat.js
Refactor concat.js

Changed:
- Replaced "fs" callback API with "fs/promises" promise API to centralize catching of errors and easier readability of file.
- Prefixed "fs/promises" with 'node:' URI scheme to target Node.js builtin modules.

locomotivemtl/locomotive-boilerplate,stephenbe/locomotive-boilerplate,stephenbe/locomotive-boilerplate,locomotivemtl/locomotive-boilerplate,stephenbe/locomotive-boilerplate