Explore support for VSCode's new Multi-root workspace feature.
Notably, rootPath now can be a list.

https://github.com/Microsoft/vscode/wiki/Extension-Authoring:-Adopting-Multi-Root-Workspace-APIs

This is somewhat different from #385, depending on the way the user uses VSCode.
For the following app,

/app
  /client
  /server
  /doc

• One might open /app in one Window. Vetur support for /client is Support sub folders and multiple tsconfig files in one window #385.
• One might open /app/client, /app/server as two top-level folders. That's this issue.
• One might open two Vue apps as two top-level folders. That's this issue.