Transitions / animations

Hello, this is a feature request, and I guess it's not an easy one :)

It would be great to be able to edit, translate, rotate or scale an existing shape dynamically, and interpolate this transformation over a custom amount of time.

Ex:

```
let ell = new Zdog.Ellipse({
addTo: illo,
diameter: 80,
translate: { z: 40 },
stroke: 20,
color: '#636',
});

ell.animate({
diameter: 150,
translate: {z: -40},
transition: "500ms",
});

// Result: the ellipse will change diameter from 80 to 150 and move from 40 to -40 in 500ms.
```

Bonus points if you manage to implement transition-timing-function too (linear, ease-in-and-out, etc)

Thanks, and congrats for this lib!

The issue provides a JavaScript example using Zdog.Ellipse and animate(). Start by locating the existing shape-property and render/update entry points, then trace how animation timing is handled. Done means shape transformations interpolate over a requested duration, with the listed timing functions supported if feasible, and the behavior is covered by tests.

Written by the indexing model from the issue text.