Custom bundle doesn't generate CSS file anymore.

When generating a custom bundle, no CSS file is generated any more, even though it's mentioned in the output.

Steps to reproduce

$ git clone https://github.com/plotly/plotly.js.git
$ cd plotly.js
$ git checkout e6b155c1c91603eb0946f111bc3415b222422230
$ npm install
$ npm run custom-bundle -- --traces sunburst --transforms none --strict --out sunburst
> plotly.js@2.35.2 custom-bundle
> node tasks/custom_bundle.mjs --traces sunburst --transforms none --strict --out sunburst

{
  traceList: [ 'scatter', 'sunburst' ],
  name: 'sunburst',
  index: '/users/tkm/dhauck/kekse/plotly.js/lib/index-strict-sunburst.js',
  strict: true,
  dist: '/users/tkm/dhauck/kekse/plotly.js/dist/plotly-sunburst.js',
  distMin: '/users/tkm/dhauck/kekse/plotly.js/dist/plotly-sunburst.min.js'
}

  dist/plotly-sunburst.min.js    1.0mb ⚠️
  dist/plotly-sunburst.min.css  63.9kb

⚡ Done in 750ms

This results in the following extra files being generated:

$ git clean -dxn
Would remove dist/plotly-sunburst.min.js
Would remove lib/index-strict-sunburst.js
Would remove node_modules/

从 tasks/custom_bundle.mjs 开始，并在提交 e6b155c1c91603eb0946f111bc3415b222422230 中重现文档所述的 npm run custom-bundle 命令。将生成的 dist/plotly-sunburst.min.js 和预期的 CSS 输出与 git clean -dxn 进行比较；当 custom bundle 生成构建输出中提到的 CSS 文件时，即表示完成。