Flatten public packs directory

[aduth]

🛠 Summary of changes

Flattens the public/packs directory to avoid nested public/packs/js.

Why?

• There are only JavaScript packs, so it's redundant to further qualify with a js/ folder
• As a continuation of previous efforts Use SHA256 for JavaScript subresource integrity #10647 Optimize preload response headers to prioritize critical assets #10612, this marginally reduces path size for asset references to help fit within Rails' 1kb hard cap for resource hints

Alternatively, I had considered flattening to public/js instead of public/packs, both to be more accurate to what it contains and to save a few more characters. This would require additional effort, since platform code exists which references public/packs/manifest.json and would need to be updated, in addition to corresponding internal references.

📜 Testing Plan

Verify no regression in JavaScript interactivity.