Assign script output name in ESBuild command

[aduth]

🛠 Summary of changes

Updates JavaScript build script to perform the rename of auto.js source script to main.js dist script as part of the ESBuild command itself, using custom output path support.

This is both simpler, and will hopefully address some inconsistent issues where the post-build script isn't always run as expected, resulting in a scenario where main.js 404s.

📜 Testing Plan

1. npm start
2. Visit http://localhost:4000
3. Observe no errors in the developer tools console, and script loading works as expected (e.g. banner is not expanded by default)