This changes the cssparser setup to:

 * Avoid having to do copies of the ParsingContext all over the place,
   which is useful because I plan to stash more nesting state in there.

 * Use the new RuleBodyParser which allows parsing qualified rules,
   declarations, and so on. Though we still don't use this anywhere.

The next step is to join NestedRuleParser and PropertyDeclarationParser,
so that we can parse declarations in a lot of the nested rules as well.

Differential Revision: https://phabricator.services.mozilla.com/D178053