JetBrains: Feature: Add our code intel to IntelliJ

Created by: vdavid

For the virtual files downloaded from Sourcegraph and opened in the editor, currently, the IDE's code intel is active.

This in-IDE code intel is sufficient for mapping the local relations, but it doesn't have access to the rest of the files in the repo (and other repos and NPM packages), so it can't do cross-file definition jumps and reference searches. We could provide that by injecting our code intel into the IDE.

This would be a major feature that would need a lot of investigation and testing.