Note and Warning support in Markdown

GitHub now renders notes and warnings differently. Please support this behaviour.

The exact colors and looks in light and dark modes will have to be taken into consideration.

The icons appear to be inline SVGs, so they can be directly copied from the Elements tab in the browser DevTools.

Syntax:
```
> **Note** // This is case sensitive. 'notes', 'note' or any other variation will not work.
>
> This is a note.

> **Warning**
>
> This is a warning.
```

Render:
> **Note**
>
> This is a note.

> **Warning**
>
> This is a warning.

Use the provided Note and Warning syntax as the rendering case, and locate the preview's Markdown rendering and styling entry points. Compare the output with GitHub in light and dark modes, including the inline SVG icons; done means exact-case Note and Warning blocks render with matching colors, layout, and icons.

Written by the indexing model from the issue text.