Add item date / time

[noyainrain]

Add optional time attribute to Item.

Draft:

class List:
    """
    .. describe:: features

       Available features are ``check``, ``vote``, ``value``, ``time``, ``location`` and ``play``.
    """

class Item:
    """
    .. describe:: time

       *datetime* or *date* of the item. May be ``null``.
    """

    time: date | None

# Future reference

class Item:
    """
    .. describe:: end_time

       End *datetime* or *date* of the item. May be ``null``.
    """

    end_time: date | None