Support mermaid configuration via front matter

[multimeric]

Description

Mermaid's recommended way of applying configuration is via frontmatter for the diagram.

However, if we create a Quarto document that has mermaid frontmatter:

```{mermaid}
---
title: Hello Title
config:
  theme: base
  themeVariables:
    primaryColor: "#00ff00"
---
flowchart
	Hello --> World
```

We get an error: ERROR: Expected YAML front matter to end with '---'

[cscheid]

If you're getting that YAML parsing error, it's a Quarto bug. I'm reluctant to promise a quick fix because it boils down to the way Quarto currently implements frontmatter detection (we inherit the bad behavior from knitr where they're detected with regexes). This bug will 100% be fixed when we switch to https://github.com/quarto-dev/quarto-markdown, though! (The planned timeline is an opt-in mechanism in 1.9, and an eventual switch to quarto-markdown being the default behavior.)

With all that said, I'm having a hard time reproducing your report. I see this:

output

input

---
format: html
---

## Hello

```{mermaid}
---
title: Hello Title
config:
  theme: base
  themeVariables:
    primaryColor: "#00ff00"
---
flowchart
	Hello --> World
```

Oh no.

(The fact that we override mermaid's theming is a separate bug.)

[multimeric]

You're right, this isn't as reproducible as I thought. I'll re-open this if I hit a more general parsing error.