Document the “show” directive for JavaScript fenced code blocks

[mbostock]

We don’t currently document that you can use the show directive to echo the contents of a live JavaScript code block.

```js show
1 + 2
```

We might also want to call it “echo” for parity with Quarto.

[mbostock]

Depends on #28.

[mbostock]

It’s called echo now, and added.