[Feature] Frontmatter/metadata support

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

Some programs parse document metadata (or "properties") inside a triple-dash delimitated front matter section. For example, the following properties are example configuration parameters for a Marp slideshow:

---
theme: default
class: invert
auto-scaling: true
---

Milkdown doesn't know what to do with this at present:

Solution

Some editors just provide an extra-formatted YAML editor:

Obsidian really spices things up:

Alternatives

Use a different editor other than Milkdown