Set up dev asset build pipeline

[ozydingo]

Rather than running npm build to run manual tests it would be great to integrate a watch / build / start like mechanism to serve the static assets. Since the gem depends on the built assets being served from Rack, this means either (a) finding the output directory these processes use or emulate them with our own build/watch process, or (b) redirecting to the dev server on request, which might be done by adding dev-setup-awareness to the rack app.