local media assets are not promoted to file attachments in hypertext literals

[cinxmo]

Local media assets should be promoted to file attachments in hypertext literals.
File structure:

├── docs
│   ├── sub
│   │   ├── pic.jpg
│   └── test.md

In test.md:

```js
const testArray = ['a', 'b', 'c'];
```

```js
function formatLetter(letter) {
	return html`
		<div>
			<img src="./sub/pic.jpg">${letter}</img>
		</div>
	`;
}
```

<div>
	${testArray.map((t) => formatLetter(t))}
</div>

Error:
Running yarn dev and navigating to http://127.0.0.1:3000/test shows a 404 when fetching http://127.0.0.1:3000/sub/pic.jpg.

Expectation:
The fetch for http://127.0.0.1:3000/sub/pic.jpg should be rewritten as http://127.0.0.1:3000/_file/sub/pic.jpg

[mbostock]

This isn’t a fixable problem in our current design: we can only promote assets that are declared statically, and anything within a hypertext literal is (by definition, since it’s inside JavaScript source rather than HTML) dynamic. You need to use FileAttachment if you want these assets to be treated as file attachments. I noted this previously in #424 (comment) and probably should be something we document.

(We could try to static analyze html tagged template literals of course, but my point is that you could always have JavaScript that is purely dynamic, so we’re never going to be able to find all the referenced files through static analysis. Therefore I suggest that we don’t try to static analyze beyond what we already do with FileAttachment.)