Move away from using BrowserWindow for source view

[pestevez]

The Source view uses webview with the view-source: prefix applied to the URL to get a formatted view of the Instant Articles markup.

This implementation gets the jobs done well, but it is preventing us from storing in the client the response from the preview webserver and forces us to to a loadURL. We should try to replace the BrowserWindow (webview) with an HTML view of the formatted code.

There ara multiple libraries that we could leverage, like Prism. I can think of a few approaches:

1. We show the unformatted code and the library used takes care of the formatting using JS and CSS.
2. We format the code from Node (for example, using prismjs and display the formatted text.
3. We use a pre-existing React component that displays formatted code (without using a webview).

I found this tutorial that describes something similar https://betterstack.dev/blog/code-highlighting-in-react-using-prismjs/