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

Start by reading the linked CSS Conditional Rules working draft and inspecting the existing postcss-conditionals project for relevant behavior. Determine whether the draft can be implemented as a PostCSS transform and define the scope of a new condition-rules plugin; done means the agreed rules are implemented and verified against the specification.

Written by the indexing model from the issue text.