JetBrains: Improve file preview

Created by: philipp-spiess

The file preview panel is currently very simple and only shows the file contents. There are a few improvements that we should make before shipping this to more people based on our already established designs on Figma:

• Show the filename and reponame
• Add some spacing below it to explain how to open the file

Additionally, this is not part of Figma but quite confusing to me at the moment, we should add some sort of loading indicator for when you navigate between results. We currently show the old preview until we have fully downloaded the new file and it's unclear to a user that work for that is happening in the background.