Conditional Rules

### What would you want to propose?

Conditional Rules spec is now a working draft.
We could consider adding it.

I still need to read through the spec fully to know if this can be done through PostCSS transforms.

https://www.w3.org/TR/2021/WD-css-conditional-5-20211221/

### Suggested solution

new plugin for condition rules.

### Additional context

https://github.com/andyjansson/postcss-conditionals

### Validations

- [X] Follow our [Code of Conduct](https://github.com/csstools/postcss-plugins/blob/master/CODE_OF_CONDUCT.md)
- [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

### Would you like to open a PR for this feature?

- [X] I'm willing to open a PR

まず、リンクされた CSS Conditional Rules の作業草案を読み、既存の postcss-conditionals プロジェクトを調査して関連する動作を確認します。草案を PostCSS の変換として実装できるかを判断し、新しい condition-rules プラグインの範囲を定義します。合意したルールが実装され、仕様に照らして検証されれば完了です。

索引モデルが issue の本文から書いたものです。