Add Next.js Built-in CSS support

[p-chan]

Changes and Fixes

• [Fix] Add Next.js Built-in CSS support

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Note (en)

1. Next.js upgraded from 9.1.1 to 9.4.4 and updated from @zeit/next-css to Built-in CSS
2. Then, image with something like url('*.png'), is converted to a ./.next/static/media/*.png.
3. Moreover, the contents of the file are stored in the module.export = data:image/png;base 64... or something like that.
4. Of course, the image won't show up because it can't be properly referenced from the browser.
5. I fixed it based on these
   a. SCSS modules cannot load images correctly with next-images package vercel/next.js#11164
   b. cyrilwanner/next-optimized-images@67697bb
   c. cyrilwanner/next-optimized-images@e5f656e

Note (ja)

1. 私は自分のプロジェクトを Next.js 9.1.4 から Next 9.4.4 にアップデートするとともに、@zeit/next-css から Built-in CSS に移行しました
2. すると、CSS 内の url('./*.png') のような記法で読み込んだ画像が . /.next/static/media/*.png に書き出されるようになりました
3. しかも、ファイルのコンテンツは module.export = data:image/png;base 64... のようになっています
4. もちろん、ブラウザから正しく参照できないので画像は表示されません
5. 私は以下を参考に修正しました
   a. SCSS modules cannot load images correctly with next-images package vercel/next.js#11164
   b. cyrilwanner/next-optimized-images@67697bb
   c. cyrilwanner/next-optimized-images@e5f656e

[jagaapple]

Is this backward-compatible with previous version? (i.e. @zeit/next-css )
Otherwise, could you consider adding to a new option?

[jagaapple]

This approach has been adopted in #38 . Thank you.