onStep callback for Motion

I'm curious how maintainers would feel about adding a callback for each step (more or less calling an optional callback each time the `render` function of `Motion` is run). We use `Motion` at my work and have found ourselves in a situation where `onStep` (or something like that) would be really helpful.

Please let me know if this is something we're open to. I'd be happy to do the work myself. If it's a no go, I'm happy to fork.

Cheers!

Start by reviewing the Motion component's render function, which the issue identifies as the point where an optional callback would run. Clarify the callback's API and step semantics with maintainers; done means an agreed onStep behavior is implemented and verified for each render step.

Written by the indexing model from the issue text.