Support passing `ref` on children

[neoncube2]

I just spent half a day trying to figure out why my ref wasn't working, because I had it on the inner element instead of on <Popover>, haha XD

e.g., I had this:

<Popover
  ...
>
  <button ref={(newRef) => console.log(newRef)}>
    test
  </button>
</Popover>

But needed this:

<Popover
  ref={(newRef) => console.log(newRef)}
  ...
>
  <button>
    test
  </button>
</Popover>

I think it'd be nice if ref were supported in either place. It looks like this would be as easy as just modifying this line of code in Popover.tsx so that it merges any existing ref attributes:

const renderChild = () => cloneElement(children, { ref: handleRef });

I'd be happy to try to provide a PR, if you like; just let me know :)

[neoncube2]

I think we might be able to just use mergeRefs() from react-router, since its code is also MIT licensed: https://github.com/remix-run/react-router/blob/7b0418112607223ab49ea28b2deea4297bf60fe9/packages/react-router/lib/dom/lib.tsx#L644