Whether or not this chip is enabled for input.
If this is true, but all of the user action callbacks are null (i.e.
DeletableChipAttributes.onDelete), then the
control will still be shown as disabled.
This is typically used if you want the chip to be disabled, but also show a delete button.
For classes which don't have this as a constructor argument, isEnabled returns true if their user action callback is set.
Defaults to true. Cannot be null.
final bool isEnabled