Error: Export fails when using characters outside Latin1 range in a Mermaid diagram

[maezawa-akira]

Thank you for developing this awesome plugin!!

Issue

Export using pandoc fails when using non-ASCII characters in a Mermaid diagram, returning the error: Pandoc Export Failed: InvalidCharacterError: Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.

Minimal reproducing example

```mermaid
graph LR
    G[こんにちは] --> A
```

The problem seems to be in convertSVGtoPNG() in renderer.ts, which calls btoa() on a SVG data that contains non-Latin1 characters.