feat: add undefined type option

[wusteven815]

• Closes #549
• Add a new Undefined object for nullable props
  • Nullable props can be declared with _nullable_props passed to component_element/BaseElement
  • For these props, None will be translated to null and Undefined will not exist in the props object

[wusteven815]

Example:

from deephaven import ui

@ui.component
def nullish_calendar():
    return ui.flex(
        ui.calendar(value=ui.types.Undefined),
        ui.calendar(value=None),
    )

calendar_test = nullish_calendar()

• For Undefined, the calendar can still be edited, while for None, the calendar can not be edited