Implement the nested declarations rule.
I didn't put this behind a pref because it was a bit annoying to do with
the parser changes while keeping it fast, and because, at least nesting
wise, this is unlikely to cause compat issues.
There are some complexities for @scope, which I think I got right.
Fixed the tests not to depend on @scope necessarily per the discussion
in web-platform-tests/interop#697.
Differential Revision: https://phabricator.services.mozilla.com/D222817
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1918408
gecko-commit: 0aa1570bfecae9690855f6d6594e79f59593e36c
gecko-reviewers: dshin, webidl, smaug