Animate interval

Use the animateInterval attribute to package multiple boards as 1 SVG which transitions through each board at the specified interval (in milliseconds).

src/content/docs/example.md

```d2 animateInterval=1000
How does the cat go?

layers: {
  cat: {
    Meooooooooowwww
  }
}
```

The above code block will be rendered as the following diagram with an animate interval of 1 second: