feat: collab support in vscode-workbench extension

[Vtec234]

This PR adds support for collaborative editing to the vscode-workbench extension. The design I settled on is as follows.

• The project directory is write-mounted in every VSC server (editor session). VSC servers open, read, and write project files directly.
  • Conflicts resulting from concurrent writes would normally produce an error in VSC. We turn this off by setting files.saveConflictResolution.
• Text documents that clients have opened are synced through our collab-server. Edit rebasing is done by Yjs.
  • Collab-server picks up file contents from disk when initializing collaborative state, but stores the collaborative state in a database afterwards.
    • This leaves some potential for divergence between the on-disk contents of project files and the collaborative state. Divergence is not substantial when users auto-save (the default in VSC-Web), but we should add more safeguards in follow-up PRs.
    • Collaborative state is destroyed as soon as all VSC servers exit (defaults to a few hours after client disconnect, afaict). We could choose to persist it longer if desirable.
      • [ ] Persist DB. In feat: awareness for collaboration #20 we save document contents to disk when collab-server exits - there may be no need to persist the edit history in the Yjs CRDT also.

[Vtec234]

Prisma might be overkill for this simple database after all..

[jcreedcmu]

Agree

[jcreedcmu]

We discussed the design a bit in a call, and made the following diagram: