CSS Values lvl 5, standard if() syntax

### What would you want to propose?

Spec: https://drafts.csswg.org/css-values-5/#if-notation
This is the draft for a standard conditional selector.
There is a plugin that does similar thing called postcss-conditional-values, it could either be modified to support new experimental syntax, or a new plugin could be made partly based on it.
Would be nice to have a polyfill for this in the future with the standard syntax.

### Suggested solution

Spec allows media queries, and overall it is differs from postcss-conditional-values in a number of ways, so a new plugin is probably better.

Spec allows stuff like this:
```css
.valid-if {
--if-clause: media(width >= 600px): blue;
color: if(...var(--if-clause); else: green;);
}
```
I don't think is something that can be polyfilled

### Additional context

Overall this is more of a different syntax for queries, and doesn't replace postcss-conditional-values.

This request is related to https://github.com/csstools/postcss-plugins/issues/134 for some part.

### 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?

- [ ] I'm willing to open a PR

まず CSS Values Level 5 の if() 記法の仕様を読み、既存の postcss-conditional-values plugin と比較してください。この issue では、ファイル、テスト、定義済みの実装範囲が指定されていません。完了とするには、新しい plugin が標準構文の有用なサブセットをサポートできるかどうか、およびその範囲をどのように検証すべきかについて合意する必要があります。

索引モデルが issue の本文から書いたものです。