fix(datepicker): ensure flatpickr instance is crated after lazy input resolves

[tay1orjones]

Closes #10692

If the DatePicker was brought in via React.lazy, the ref assigned to the start input was not being updated after the component resolved. This caused the flatpickr instance to never be assigned to the input, so the calendar would never show.

Changelog

Changed

• update datepicker to use useCallback instead of useRef, store the status of the ref in state, and use it to trigger the existing useEffect after react.lazy resolves

Testing / Reviewing

• Check out the simple datepicker story, it should function normally. I've modified it to use react.lazy
• Don't merge this PR until we've reverted the story to it's original state. Done

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[francinelucca]

😍 LGTM!