Refine type annotation for Date element

[daya0576]

This PR refines the static type annotation for Date element, as the param value accepts different type of inputs:

ui.date(value='2023-01-01', on_change=lambda e: result.set_text(e.value))
ui.date({'from': '2023-01-01', 'to': '2023-01-05'}).props('range')
ui.date(['2023-01-01', '2023-01-02', '2023-01-03']).props('multiple')
ui.date([{'from': '2023-01-01', 'to': '2023-01-05'}, '2023-01-07']).props('multiple range')

[daya0576]

Hi @falkoschindler, could u help review this PR, since I'm unable to add reviewers for it.

[falkoschindler]

Thanks for the pull request, @daya0576!
Sure, it's on my list. I just haven't found the time to look into it. But I'll review it shortly.

[falkoschindler]

Thanks again for your contribution! No matter the size, every change helps us improve and refine the project. Your effort is truly appreciated! 🙂