Handle the export command while non-file schema

[yhatt]

The export command assumes to save into local file (file:///c:/xxx). On the other hand, Marp for VS Code is also working on remote container (e.g. vscode-remote:// schema such as Docker container, SSH, and WSL). In this case, the export command may try to save into the host filesystem vscode-local:// which is not known by CLI if selected Show locals in the save dialog.

We should consider one of them:

1. Simply output error if detected non-file schema in the path for save.
2. Handle the path to the host of container (Export to a temporal file and save to the host through VS Code API)