def add_area(vector):
    """
    Function to add area as a property to a GEE feature collection representing a polygon or multipolygon.

    :param vector: GEE feature collection representing a polygon or multipolygon feature.
    :return: GEE feature collection of a polygon/multipolygon with an area property for each feature
    """
    area = vector.geometry().area(maxError=1)
    return vector.set('area', area)