feat(Slot): mergeProps handlers should return the value of the child handler

[Sheraff]

Description

In some cases, we can return values from the event handlers. The best use-case I have for this is the following:

<Button
  onClick={() => mutateAsync({...})}
>

In a case like this, we can use the fact that mutateAsync returns a Promise to automatically turn on a "loading" state on our button, and turn it off when the promise resolves/rejects.

But if Slot doesn't "forward" the return value of the event child handler, this use-case becomes impossible if Button is in a Slot (that also applies an onClick handler)

<Tooltip>
  <Button onClick={() => mutateAsync({...})} />
</Tooltip>

This PR proposes that Slot should return the return value of the child handler. This makes Slot even more "transparent" to developers using it.

---

• 📝 Use a meaningful title for the pull request and include the name of the package modified.
• ✅ Add or edit tests to reflect the change (run yarn test).
• 🔍 Add or edit Storybook examples to reflect the change (run yarn dev).
• 🙏 Please review your own PR to check for anything you may have missed.

If you like the idea of this PR, I can try and think of tests / storybook examples