Handle export into non-file scheme

[yhatt]

The path for exporting slide may become non file scheme (e.g. vscode-local if used together with Remote extension). This PR is adding support of the export command for non-file scheme.

• If the output path had file scheme, Marp CLI tries to export into specified path as same as current.
• If the output path had other URI scheme, Marp CLI tries to export into tmpfile, and copy it to specified path via vscode.workspace.fs.copy.
  • In this case, Marp for VS Code will not try to open the output file after exporting.
  • When the URI scheme has marked as readonly, Marp for VS Code will notify users that and not try to export.

An exported slide that has processed in the remote container can save into local file system correctly.

Resolve #262.