Promote img, video, audio, and picture elements to file attachments

[mbostock]

We currently detect link elements with href attributes and promote them to file attachments, but we should do the same with other types of elements that commonly reference static files.

https://github.com/observablehq/cli/blob/0081db4f685ef635df42d0725ec2bbac5e87fa6c/src/markdown.ts#L64

[Fil]

Adding img[src] in normalizePieceHtml works well:

  for (const element of document.querySelectorAll("img[src]") as any as Iterable<Element>) {
    const src = element.getAttribute("src")!;
    if (/^(\w+:)\/\//.test(src)) continue; // absolute url
    if (canReadSync(join(root, src))) {
      context.files.push({name: src, mimeType: mime.getType(src)});
      element.setAttribute("src", `/_file/${src}`);
    }
  }

likewise a[href] is covered by extending the selector above to "link[href],a[href]".

Can we list what we'd want to support?

• img[src]
• img[srcset] and it peculiar syntax
• picture source[srcset]
• video[src]
• audio[src]
• audio source[src]
• video source[src]
• a[href] (for example, <a href=links.md>view source</a>)

(speaking of srcset, would we want to allow a function to generate reduced images for different srcset on build? would this be some sort of plugin which would hook into/before/after normalizePieceHtml?)