[Feature] Frontmatter/metadata support

[batonac]

Initial checklist

• [x] I agree to follow the code of conduct
• [x] 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 😀

[lloydjatkinson]

+1 for this. Even the option of ignoring/not rendering the frontmatter YAML would be a good interim option?