Feature request: support MyST directives and roles

MyST (Markedly Structured Text) is a reStructuredText-inspired superset of CommonMark. It comes with two major additions: [directives and roles](https://myst.tools/docs/mystjs/syntax-overview):

- **Directives** are code blocks with a special attribute syntax:
```` markdown
``` {directiveName} arg1 arg2
Contents
```
````
- **Roles** are similar to the attributes extension, but come before the inline code:
```` markdown
{role}`text`
````

MyST is [supported](https://jupyterbook.org/en/stable/content/myst.html) by the Jupyter Book project.