`@scope` support

### What would you want to propose?

I'd like to suggest support for `@scope`

* https://drafts.csswg.org/css-cascade-6/#scoped-styles
* https://developer.mozilla.org/en-US/docs/Web/CSS/@scope
* https://developer.chrome.com/docs/css-ui/at-scope

### Suggested solution

This is just an idea, I have no idea if this would work in general.

https://jsfiddle.net/mindplay/agd9k2p1/

So this:

```css
@scope (.panel) to (.content) {
h2 {
color: red;
}
}
```

Becomes this:

```css
:where(.panel) h2:where(:not(.content h2)) {
color: red;
}
```

And a more complex selector like this:

```css
@scope ([scope=panel]) to ([scope]:not([scope=panel])) {
h2 {
color: red;
}
p {
font-weight: bold;
}
}
```

Becomes this:

```css
:where([scope=panel]) h2:where(:not([scope]:not([scope=panel]) h2)) {
color: red;
}

:where([scope=panel]) p:where(:not([scope]:not([scope=panel]) p)) {
font-weight: bold;
}
```

So this would require both `:where` and `:not`, I think - to get the right specificity.

But this is just a rough idea - I'm not a CSS superhero, and I don't know if this is completely correct or scoped properly etc.

### Additional context

Only a few browsers support this already.

https://caniuse.com/css-cascade-scope

But this would be an incredible feature, if we didn't have to wait 3-5 years for every browser to support it - we could basically stop using CSS frameworks, CSS-in-JS, Tailwind etc. would all essentially be obsolete (from my personal point of view!) and we could finally have something resembling component-scoped CSS natively in the browser *without* JavaScript. 😄

### 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

Das Issue nennt kein Plugin, keine Datei und keinen Test-Einstiegspunkt. Beginne damit, die Plugin-Struktur des Repositorys und die bestehende Verarbeitung von CSS-At-Regeln zu untersuchen, und lies anschließend die verlinkte CSS-Cascade-6-Spezifikation und Browser-Referenzen. Erledigt wäre die Aufgabe, wenn ein abgestimmtes Implementierungsdesign für @scope, Kompatibilitätsgrenzen und Tests vorliegt, die das vorgeschlagene Verhalten abdecken.

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