Define whether supports a virtual workspace

[yhatt]

https://code.visualstudio.com/updates/v1_56#_define-whether-your-extension-supports-a-virtual-workspace

Call to Action: Please check whether your extension can handle virtual workspaces, and set the virtualWorkspaces capability accordingly in your package.json.

For supporting virtual workspace

➡️ #225

Memo

Marp extension is still using Node.js fs. It would be resolved by replacing to use VS Code workspace API.

The core problem is that slide export is depending on Marp CLI and it's using fs internally. It requires to change Marp CLI API, to accept alternative fs module.

Even so, exports depending on Puppeteer are not working because Chrome browser won't recognize files in VS Code virtual workspace. We may have to restrict export type to HTML export when the extension is working on a virtual workspace.