Detect Webpack for Rails 7 installs

[unikitty37]

What is the purpose of this pull request?

Improve Webpack detection.

Rails 7 replaces the webpacker gem with jsbundling-rails, so the config/webpack folder is not created; instead, it puts webpack.config.js in the project root.

What changes did you make? (overview)

Changed the USE_WEBPACK definition to check if either is present.

Is there anything you'd like reviewers to focus on?

Checklist

• [ ] I've added tests for this change
• [X] I've added a Changelog entry
• [ ] I've updated a documentation

[palkan]

Thanks!