Update Files section in sidebar to reflect sourcegraph.com's files section

Created by: jjinnii

User Problem: Because the files section is identical to VS Code's native file view, users do not understand that our files section is specific to the Sourcegraph extension. It also creates the impression that files opened from search results are somehow downloaded locally and editable.

Solution: Visually communicate that the files section is the Sourcegraph extension files section. Designs have been updated to mirror Sourcegraph.com's files section.