Wait for existing workspace start

[code-asher]

If you have two VS Code windows open, for example, and they both try to start the workspace, one will fail with a 409. If we get this we should wait for the in-progress build instead of erroring.

I imagine you could get a similar issue if you started the workspace from the dashboard or the cli and then launched VS Code immediately after or simultaneously, although this might not reproduce as naturally.

Currently blocked because we might implement this in the API.

https://github.com/coder/coder/issues/13032

[code-asher]

Related: https://github.com/coder/vscode-coder/issues/243

The decision here will also affect the JetBrains plugin: https://github.com/coder/jetbrains-coder/issues/401