Prettify the display of the _source field in discover

If you have no columns selected to display in Discover, it displays `_source_ by default. It's difficult to scan by eye, so after some discussion, we decided to add some prettiness to distinguish between fields and values, and make it easier on the eyes.

I had added some search highlighting functionality but ultimately decided to pull it out until it's fully baked. I think this should get reviewed on its own anyway.