Consolidate and clean up `*item()` methods for `ItemsMixin` nodes

[Vollkornaffe]

Currently, we still have to assert isinstance(item.output, BooleanSocket), but it's always a boolean socket:

[BradyAJohnston]

Thanks for highlighting - should be easy enough to type the items for this as well.

[BradyAJohnston]

Mentioning here for consistency with the Discord chat - the item() version is typed.

MenuSwitch.boolean().item().output # property typed

The issue currently is that there are several methods for creating items, with add_item() being generic and coming from the ItemsMixin and the item() being typed and defined on the individual nodes themselves.

I'll update the description for this issue - for v530 we can consolidate the API methods around the item() and ensure everything is typed.