Frontmatter support

[delaneyj]

Coming from other tooling (such as sli.dev) it looks like standard frontmatter support is not supported. https://mystmd.org/guide/frontmatter for example shows what others consider frontmatter contents. I would like to use the parser to create a KV of the contents while still using the rest of the document to generate HTML

---
title: Welcome to my Game!
description: Let's get started
spoken_by: Narrator
spoken_to: Player
---

# Welcome

some normal content