Determines the order to lay children out horizontally and how to interpret
end in the horizontal direction.
Defaults to the ambient Directionality.
If the direction is Axis.horizontal, this controls the order in which the children are positioned (left-to-right or right-to-left), and the meaning of the mainAxisAlignment property's MainAxisAlignment.start and MainAxisAlignment.end values.
If the direction is Axis.horizontal, and either the mainAxisAlignment is either MainAxisAlignment.start or MainAxisAlignment.end, or there's more than one child, then the textDirection (or the ambient Directionality) must not be null.
final TextDirection textDirection