feat: collaboration server

[Vtec234]

• Add Yjs-based collaboration server that sync changes between clients. (No integration in VSCode in this PR.)
• Start the collab server on-demand when editor servers start.
• Fix race condition causing clients to see 404 from Nginx when the editor server has already started listening on a port, but Nginx isn't forwarding to it yet.
• Try to eliminate other (theoretical, not observed) race conditions in editor server/collaboration server management.
• Auto-enable VSCode extension host debugging in dev mode.
• Make directories in the container match the host more closely.
• Build process fixes.