Stop rebuilding the counter style cache from every style update.
That made unrelated restyles pay the full counter-style cost even when
no relevant stylesheet state had changed.
Dirty the cache when stylesheet rule caches are invalidated and rebuild
it on the first counter-style lookup instead. Also make cold cache
rebuilds include user stylesheets.
Add regression tests covering insertRule() and replaceSync() updates
that should make newly defined counter styles take effect.
The spec says that the `system` descriptor defaults to `symbolic` but
previously we just ignored any `@counter-style` rules without a `system`
descriptor
