feat!: emit CSS files to dist instead of dist/static/css

[chenjiahan]

Summary

Emit CSS files and async files to dist instead of dist/static/css.

• Align with distPath.js.
• Usually library authors prefer flatter output instead of nested output.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[codspeed-hq]

CodSpeed Performance Report

Merging #460 will not alter performance

_{Comparing css_dist_folder_1121 (2e16c59) with main (39ba4d9)}

Summary

✅ 5 untouched benchmarks