feat: add undefined type option

[wusteven815]

• Closes Python - Differentiate null from undefined #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

Added

[bmingles]

Code changes look good to me. I like how this approach simplifies things. Left 1 comment to update comment block with new _nullable_props arg. Also would be good to put something about this in the UI docs.

[mofojed]

@wusteven815 as @bmingles suggested let's add a couple docs for this as well.

• An example to picker.md showing the use of None vs Undefined
• A subsection in architecture.md components, named "Props" where we detail how we're mapping React JS props to Python args in general (children are mapped to positional args, other props are mapped to keyword args, along with a couple small examples), then a subsection within that for "Handling null vs. undefined" and detail how the issue (JS has null/undefined, some components require that differentiation, Python only has None), and how we're addressing it.

Added

[bmingles]

Suggested changes for controlled vs uncontrolled description / examples.

Added

[bmingles]

LGTM