Add lint rule to ensure `<img>` is self closing about design

[yaili]

@khiga8 does this happen in the design docs only, or across all primer.style docs?

[khiga8]

This was specifically observed in this design repo in a .mdx file. Here's a related commit that fixed it.

[koddsson]

JSX isn't HTML and has its own semantics. One of the ways it differs from HTML is that all elements need to close, either by self-closing or with a closing element.

<img> is invalid in JSX but is how we define images in HTML.