add reflect to property or attribute

[bennypowers]

There should be a way to signal whether or not a property reflects to it's associated attribute when set

[justinfagnani]

I think to do this the way we've done other features where a custom element adds interface on top of classes we should introduce a CustomElementField that extends a ClassField:

interface CustomElementClassField extends ClassField {
 attribute?: string;
 reflects?: boolean;
}

Then change CustomElement to override members:

  members?: Array<CustomElementMember>;