Treat a scoped `@import` rule as a scope descriptor while traversing
style-producing rules from imported stylesheets. Imported rules now
inherit an outer scope for unscoped imports and replace it when the
import itself carries scope(...).
Scope resolution was generalized to handle both CSSScopeRule and
CSSImportRule descriptors. Import-scope boundaries are matched using
the stylesheet that parsed the `@import` rule, while normal imported
selectors keep using the imported stylesheet context. This lets
implicit scopes, nested `@scope` rules, :scope, and top-level `&` behave
as the Cascade 6 model requires.
`@scope (a) to (b) {}` applies its contained style rules to elements
that have `a` as a parent, and do not have `a b` as a parent. Both the
`a` and `b` selector lists are optional.
Because it's situational whether a `@scope` will apply to a given
element, we store the ancestor scope on the `MatchingRule`, similar to
`@container`, and then determine during matching whether all the parent
`@scope`s match or not.
The rules for how selectors inside `@scope` are adjusted and interpreted
are a bit confusing. Unlike for other at-rules, nested style rules
inside `@scope` do not get a leading `&` added during parsing. To
support this, `adapt_nested_relative_selector_list()` now takes a flag
for whether its parent is a `@scope` or not.
`@scope` can also contain nested declarations without itself being
nested inside a style rule.
When determining their selectors, nested declarations rules adopt the
`@scope`'s scoping root if it has one, or otherwise fall back to the
parent element of the `<style>` element (not implemented here,) or the
`:root`. These are required to have zero specificity, so we wrap the
selector in `:where()`.
