Theme editor HMR

[montalvomiguelo]

Issue

During development assets cannot be loaded from our local.

Solution

This PR allows Theme Editor or Builder to load assets from our local.

1. It finds an available port
2. It uses that port to build an absolute URL on our local to serve files
3. It connects the HMR client to our local so it can handle changes inside Theme Editor or Builder
4. It reloads the page using the HMR client when files upload to Shopify
5. It updates the BRRL_PATH function, so it returns Shopify's CDN only on production

cc @wturnerharris @bengodfrey