Frontmatter first slide not working

[louwers]

Describe the bug I am trying to use frontmatter with the first slide. But it doesn't work the way I expect

To Reproduce Steps to reproduce the behavior:

1. Add frontmatter before the first slide like in the documentation https://sli.dev/guide/syntax#frontmatter-layouts
2. Instead of the frontmatter applying, I see the contents of the frontmatter on the slide:

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Slidev version: 0.49.10

[louwers]

Similarly

```yaml
# The first yaml block will be treated as the frontmatter of that slide
layout: full

Test

Results in:

<img width="950" alt="image" src="https://github.com/slidevjs/slidev/assets/649392/df535666-c043-4ae7-a95f-fb0262065b43">

[louwers]

I overlooked this:

You can configure Slidev in the frontmatter of your first slide