Cannot output the conversion result into the drive root

[yhatt]

Version of Marp Tool

Marp CLI v1.7.0

Operating System

Windows

Environment

• OS version: Windows 10
• Node.js version: 16.13.0
• VS Code version (Marp for VS Code): VS Code 1.66.1 (system setup)

How to reproduce

Try to convert Markdown into the drive root such as C:\slide.html.

Expected behavior

The converted HTML will be generated to C:\slide.html.

Actual behavior

Failed converting Markdown. (EPERM: operation not permitted, mkdir 'c:\')

Additional information

Reported by marp-team/marp#291.

It's a known pitfall that brings when trying fs.mkdirSync('c:\', { recursive: true }) into the drive root. microsoft/vscode#379