[css-cascade-6] Named scopes proposal

This issue is a follow-up to the previous issue: https://github.com/w3c/csswg-drafts/issues/9741

In that issue, I explored the way :scope works in a nested context:

@scope (.outer) {
  @scope (.inner) {
    :scope.inner .test {
      background: lightgreen;
    }
    
    /* This should never match */
    :scope.outer .test.never {
      background: tomato;
    }
  }
}

My proposal consists of two parts:

1. Augment the syntax of @scope (https://drafts.csswg.org/css-cascade-6/#scope-syntax), allowing specifying the <scope-name> as a <custom-ident> (or <dashed-ident>?), in a way similar (syntax-wise, not behavior-wise though) how we can specify a name for containers (https://drafts.csswg.org/css-contain-3/#container-rule) with a <container-name>.
2. Introduce a functional form of a :scope pseudo-class, which could be used with the scope name inside, like :scope(my-scope) (or :scope(--my-scope) if we'd decide on <dashed-ident>).

This way, we could do something like this:

@scope outer (.outer) {
  @scope inner (.inner) {
    :scope(inner) .test {
      background: lightgreen;
    }
    :scope(outer) .test {
      background: lime;
    }
  }
}

Basically, allowing specifying any specific scope defined around our selector.

Some things to think about:

• Can the same name be used multiple times? I'd say that yes, in which case, similar to anonymous scopes, the innermost one would win.
• Do we need some way to get the equivalent &, where we'd retain the scope root's selector's specificity? I'm not sure of the use-cases for that, probably could be a separate proposal in itself.

Use cases:

• Using a named scope for the root one, enabling something similar to the @at-root in Sass, or root reference in Stylus. This could work already with the :scope, but only unless a nested scope is introduced, in which case we will lose the access to the root scope.
• “Naming” scopes to be reusable inside, as a way to not repeat yourself. This way, scopes could fully replace something like BEM, where we could define a scope for the “block”, and then define sub-scopes for every “element”. Then, we retain an ability to mention “modifiers” on the block and elements separately when specifying various states and cases.

Out of this proposal's scope (sorry): naming scopes as a way to reuse their selectors: this is probably a job for CSS-extensions (label, spec; I did not find a good single issue to mention).

Start by reading the linked issue 9741 and the CSS Cascade 6 scope syntax, then compare the CSS Containment 3 container-name syntax and the CSS Extensions materials mentioned here. Done means reaching agreement on named-scope syntax, functional :scope behavior, repeated-name handling, and the relationship to nested scopes before updating the relevant specification text.