Add ability to register actions dynamically

[timc1]

Currently actions must be registered up front – though this works well for static actions; e.g. navigating pages, triggering DOM events, it falls short of easily enabling users to generate actions on the fly.

Perhaps an API like so:

const { registerActions } = useKBar(() => {...});

React.useEffect(() => {
  registerActions([
    // ...
  ])
}, [])