[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

crates/red_knot_server/src/server.rs

@@ -99,6 +99,11 @@ impl Server {
                 anyhow::anyhow!("Failed to get the current working directory while creating a default workspace.")
             })?;
 
+        if workspaces.len() > 1 {
+            // TODO(dhruvmanila): Support multi-root workspaces
+            anyhow::bail!("Multi-root workspaces are not supported yet");
+        }
+
         Ok(Self {
             connection,
             worker_threads,