Defines the appearance of the selected tab indicator.
The default, underline-style, selected tab indicator can be defined with UnderlineTabIndicator.
The indicator's size is based on the tab's bounds. If indicatorSize is TabBarIndicatorSize.tab the tab's bounds are as wide as the space occupied by the tab in the tab bar. If indicatorSize is TabBarIndicatorSize.label, then the tab's bounds are only as wide as the tab widget itself.
final Decoration indicator