improvement: Move `can?/2` and `async?/1` into `Reactor.Step` behaviour.

[jimsynz]

Makes it a little bit easier for steps to define their own capabilities at runtime.

Contributor checklist

• [ ] Bug fixes include regression tests
• [ ] Chores
• [x] Documentation changes
• [X] Features include unit/acceptance tests
• [x] Refactoring
• [ ] Update dependencies