Fix issues with `Suspense` when using DOM polyfill

While playing with Suspense here: https://github.com/Shopify/remote-dom/compare/add-react-suspense-example?expand=1, I noticed there were issues in both React and Preact. React would not render the suspense fallback at all in the worker (though it does in the iframe), and Preaect would render the fallback, but send an invalid `detach` command to the host.