File system provider docs: https://code.visualstudio.com/api/references/vscode-api#FileSystemProvider 
Example file system provider: https://github.com/microsoft/vscode-extension-samples/tree/master/fsprovider-sample and https://github.com/microsoft/vscode-extension-samples/blob/master/fsprovider-sample/src/fileSystemProvider.ts 
We'll want a mongodb document scheme for our documents so that we can manage how their read and written.

Using a file system provider will help us avoid storing files on the user's file system, improve the breadcrumbs we show on an open document (currently we show the /tmp/.../file), and improve how open documents are handled when vscode is re-opened.