Support inline expressions for attributes (and in other places)

[mbostock]

Currently inline expression ${…} are only supported for node content. So you can’t do this:

```js
const link = "https://example.com";

This is a link.

You can workaround it using [Hypertext Literal](https://github.com/observablehq/htl) like so:

````md
```js
const link = "https://example.com";

This is a ${htl.html<a href=${link}>link</a>}.

I’m not immediately sure how we would support this. It might involve writing our own Markdown parser? Or at least pre-processing the content to remove the live code expressions before passing it to markdown-it? Presumably we could repurpose parts of Hypertext Literal to parse the HTML.

[mbostock]

This is one of the trickiest outstanding questions in the CLI. If we do this, it probably requires radical changes to how we parse Markdown and generate HTML & JavaScript. We should decide whether we want to do that before Early Access.