`@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()`.
The main limitation here are that none of the container-query features
are parsed in a meaningful way; they all become `<general-enclosed>`.
Parsing for them will be added as they are implemented.
No parsing yet, just CSSContainerRule and the supporting ContainerQuery
class.
CSSContainerRule is unusual in how it matches, because instead of it
either matching or not matching globally, it instead is matched against
a specific element. But also, some at-rules inside it always apply, as
if they were written outside it. This doesn't fit well with how
CSSConditionRule is implemented, and will likely require some rework
later. For now, `condition_matches()` always returns false, and
`for_each_effective_rule()` is overridden to always process those
global at-rules and nothing else.
This is a bit under-specced, specifically there's no definition of
CSSMarginDescriptors so I've gone with CSSStyleProperties for now. Gets
us 17 WPT subtests.
