Support HMR using bundle-loader.

[jcblw]

Been looking at how we can load bundle files making it so we do not need to have one giant bundle. Not sure if this is the right place to put this, but I like the idea of this.

https://v4.webpack.js.org/loaders/bundle-loader/

Essentially we do HMR and bundle loading through this loader and that way on content scripts we can do something like this.

bundle((file) => {
  port.sendMessage({ injectBundle: file });
  // background script runs `browser.tabs.injectScript(sender.tabID, { file: message.injectBundle })`
});

We already have infra here to be able to handle some of this.