[@scope] Make '&' work like :where(scope) in @scope rules

Within @scope rules, there are currently three subtly different ways
to reference the scoping root:

 A. With the :scope pseudo-class, which matches the specific element
    that is the scoping root. The specificity of :scope is [0, 1, 0].
 B. With the '&' selector. This is really referencing the <scope-start>
    *selector* rather than a specific element [1], but it's effectively
    the same in most cases. The specificity of '&' works like
    the specificity of :is(), i.e. max across the held selector list.
 C. Implicitly, if no '&' or :scope selector is explicitly provided.
    In this case, we effectively prepend :where(scope) (+ descendant
    combinator) to the selector. The specificity of that is obviously
    zero.

This behavior is "all over the place", and although it was done
intentionally to begin with (for consistency with css-nesting),
the CSSWG has since decided [2] that '&' should now just mean
:where(:scope).

In other words, we are changing (B), while (A) and (C) remains the same.

We can carry out this change by simply not providing scoped '&'
with a parent rule pointer at all. This already has zero
specificity (covering the :where() part), and already behaves
like :scope in the SelectorChecker.

Also, I expect that we'll be able to clean up some of the special cases
around @scope in the parser once the flag is removed. For example,
when parsing relative selectors, we don't need to know the current
CSSNestingType to know whether to prepend '&' or ':where(:scope)';
we can just always prepend '&' now. This is for future CLs, however.

[1] This means that '@scope(div) { & + & {} }' can match (a <div> next
    to another <div>), but '@scope(div) { :scope + :scope {} }' can not
    (an element can not be its own sibling).
[2] https://github.com/w3c/csswg-drafts/issues/9740

Fixed: 445949406
Change-Id: I63dc55c12d875096534e69adbbd5d5298e109fa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7006094
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526767}