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.
Ah, that makes sense. I changed it in 8342ee0c036a0dca05d6caaf37a678354fdf0729