alt text support for Learn Markdown image format

[dcurwin]

I see that the latest version of VS Code has a new alt-text feature:

https://code.visualstudio.com/updates/v1_100#_generate-alt-text-in-html-or-markdown

But it only works for default Markdown image formats:

![alt text for image](../images/Introduction.png)

not the Learn Markdown format that we generally use at Learn:

:::image type="content" source="<folderPath>" alt-text="<alt text>":::

Can that support be added?