Support multi-root workspaces in Red Knot server #13052
Description
#13041 restricts the red knot server to only work when there's only one workspace opened in the editor. We should add support for multi-root workspaces.
The challenge here is that which workspace should the untitled / non-workspace files belong to.
Currently, we've made an assumption that any untitled files or non-workspace files are part of the only available workspace. When multi-root workspaces are supported, we need to make sure the user experience is still good and find a way to provide intellisense for non-workspace / untitled files when there are multiple workspaces.
One solution is to implement an ad-hoc database where these files would go but this limits the user experience because we won't be able to resolve the virtual environment for a workspace thus not able to provide any intellisense features for third-party libraries.
The implementation should also consider the fact that workspaces can be added / removed while the server is active. This is supported currently in the Ruff native server and is done via the didChangeWorkspaceFolders notification.