When we try to insert a disallowed (non-nested) statement into a
CSSGroupingRule we should throw a `HierarchyRequestError` as it being
disallowed is a "constraint specified by CSS". Previously we would rely
on `Parser::is_valid_in_the_current_context` and throw a Syntax error.
There are more constraints to be implemented.
Keep track of which CSSRule owns a CSSRuleList, and then use that to
produce a stack of RuleContexts for the CSS Parser to use.
There are certainly other places we should do this!
The regression in the "conditional-CSSGroupingRule" test is we now fail
the "inserting an `@import`" subtests differently and the subtests
aren't independent. Specifically, we don't yet implement the checks in
`CSSRuleList::insert_a_css_rule()` that reject certain rules from being
inserted. Previously we didn't insert the `@import` rule because we
failed to parse its relative URL. Now we parse it correctly, we end up
inserting it.
