Support vscode virtual file systems.

[saolsen]

Val.Town runs on Deno and there's a really great vscode extension for it by @pomdtr. https://github.com/pomdtr/valtown-vscode

Unfortionately it can't use the vscode_deno extension for the type checking, because the vscode_deno doesn't work with files in a virtual filesystem

I'm not sure what work is involved to get vscode_deno to check files in a virtual filesystem but it would be awesome to have!

https://code.visualstudio.com/api/extension-guides/virtual-workspaces