yaml frontmatter loses syntax highlighting in mdx files

[johnmathews]

Thanks for a great plugin.

I've noticed that in .mdx files yaml frontmatter doesnt have the same syntax highlighting applied as in .md files.

The difference is like the below exmampes:

.md files

---
title: >
  Photographer: Robert Doisneau
date: 2023-01-02 11:33:36
category: non-technical.snippet
tags: ['photography', 'photographer']
---

.mdx files

---
title: >
  Photographer: Robert Doisneau
date: 2023-01-02 11:33:36
category: non-technical.snippet
tags: ['photography', 'photographer']
---