JetBrains: Feature: Add repo structure to the Project tool window

Created by: vdavid

Add as a new item here, like “External Libraries”, called “Sourcegraph”, with our logo:

It'd be helpful for reviewing what you were previously looking at, and understanding and exploring the repository further.

Similar to how we do it with VS Code.

(Mentioned in this user test.) It seems to be possible based on the SDK docs