[feat]: trail onComplete event

A clear and concise description of what the feature is

A onComplete (or similar) event that triggers when ALL parts of a trail has completed. This is analogous to the onRest event on a single spring.

Why should this feature be included?

The onRest is a useful event for single springs. It would be great to have a similar event on trails.

Please provide an example for how this would work

onComplete should trigger when all springs from a single trail has reached rest.

I am achieving a similar effect by using the following code today.

    const onComplete = () => console.log('trail completed')
    const trail = useTrail(baseSquares.length, {
        to: {"rotation-x": -Math.PI / 2},
        from: {"rotation-x": 0},
        onRest: (_, w) => {
            if (w.springs["rotation-x"]._priority === offsets.length - 1) onComplete()
        },
    })

CSB https://codesandbox.io/s/quirky-visvesvaraya-8kbukr?file=/src/App.js

Start by tracing the useTrail API and its existing onRest behavior, using the linked CodeSandbox as the behavioral example. Define completion for a trail as the point when all springs from one trail have reached rest, and verify that the new event fires once for that condition.