Support virtual workspaces

[yhatt]

We're following VS Code's call-to-action. This PR will improve PDF/PPTX/image export and resolving theme CSS in the virtual workspace. Close #224.

It depends on marp-team/marp-cli#343.

Approaches

Export action

Export command is depending on Marp CLI and Chromium so the work file should always place on the local file system. Thus, I've judged the most of exist codes using fs are not required to replace.

Currently PDF/PPTX/image export (using Chromium) cannot resolve virtual assets provided by VS Code and extensions. So we're providing a simple local proxy during export process. By forcing base URL of work file to an address of the proxy, Chromium for conversion can consume presentation assets correctly.

Resolving custom theme CSS

Paths for custom theme CSS must be a relative path from the workspace. Now we are handling the case of in a virtual workspace while theme resolution. Auto-reloading CSS theme is also working even if in virtual workspace.

⚠️ FileSystemWatcher looks like supporting the custom scheme for a virtual workspace, but it requires VS Code >= 1.52.

ToDo

• Release new Marp CLI version including Allow overriding base url through baseUrl option marp-cli#343
• Update tests