expose `currentTarget` in `handleChange`

Happy to send a PR :)

From react/flow docs:

Note: To get the element instance, like HTMLButtonElement in the example above, it is a common mistake to use event.target instead of event.currentTarget. The reason why you want to use event.currentTarget is that event.target may be the wrong element due to event propagation.

Relevant code:

  handleChange = (e) => {
    const { props } = this;
    if (props.disabled) {
      return;
    }
    if (!('checked' in props)) {
      this.setState({
        checked: e.target.checked,
      });
    }
    props.onChange({
+     currentTarget: e.currentTarget,
      target: {
        ...props,
        checked: e.target.checked,
      },
      stopPropagation() {
        e.stopPropagation();
      },
      preventDefault() {
        e.preventDefault();
      },
      nativeEvent: e.nativeEvent,
    });
  };

Start in src/Checkbox.jsx at the handleChange entry point referenced by the issue, and compare the event object passed to props.onChange with the requested currentTarget addition. Verify that the callback exposes currentTarget while retaining the shown target and event methods, then run the repository's existing checks.