Frontmatter formatting errors with Stackblitz Editor on https://sli.dev/new

[normal-coder]

Describe the bug

Frontmatter formatting for multi-section Markdown is flawed at https://sli.dev/new.

Suggest adding a .prettierrc configuration to the template to avoid.

Minimal reproduction

1. Use https://sli.dev/new to initialize Slides
2. Write multi-section Markdown slides, including the following content:

... content ...

---
level: 2
---

... content ...

3. Save the document. Stackblitz’s editor will force format the document, causing the document to become:

... content ...

---

## level: 2

... content ...

Environment

• Slidev version: latest
• Browser: irrelevant
• OS: irrelevant

[kermanx]

Adding a .prettierrc to the stackblitz project does not fix this