[red-knot] Support files outside of any workspace (#13041)
## Summary
This PR adds basic support for files outside of any workspace in the red
knot server.
This also limits the red knot server to only work in a single workspace.
The server will not start if there are multiple workspaces.
## Test Plan
https://github.com/user-attachments/assets/de601387-0ad5-433c-9d2c-7b6ae5137654
