Add VS Code previewer extension

[chrisjsewell]

You can extend the VS Code Markdown previewer, via markdown-it plugins: https://code.visualstudio.com/api/extension-guides/markdown-extension

[chrisjsewell]

Implementing syntax highlighting for code cells would be a good first issue:

• https://www.npmjs.com/package/markdown-it-grouped-code-fence
• https://www.npmjs.com/package/markdown-it-fence
• Example of a markdown-it extension written in TypeScript