Make BoxyFlex Implement Flex

[caseycrogers]

Packages like gap and flutter_gutter use Flex.direction/RenderFlex.direction to determine if they are in a vertically or horizontally aligned layout.

BoxyFlex and BoxyRenderFlex should implement their Flutter counterparts so that these packages' widgets will work properly with BoxyRow and BoxyColumn out of the box(y).

Here's a related Flutter/flutter issue that if accepted would change how this should be implemented:
flutter/flutter#133394

[pingbird]

This can't be done without breaking boxy when the framework changes Flex, also, walking the tree to check the runtime type of ancestors is probably a bad idea. I'll gladly implement a directionality interface if added to the framework, I think your best bet until then is a dynamic call at layout time:

var direction = defaultDirection
try {
  direction = (parent as dynamic).direction as Axis;
} catch (e) {}