feat: Add react-native 0.78 support (#578)

In react-native 0.78, they removed `StyleLength::unit()`. Instead, you can check `StyleLength::isUndefined()` directly, which achieves the same thing.

Technically not backwards compatible, but I remember Janic saying the policy for new arch is to not be backwards compatible until it is stable and used by (pretty much) everyone.

Author: mrousavy

common/cpp/react/renderer/components/safeareacontext/RNCSafeAreaViewShadowNode.cpp

@@ -16,10 +16,10 @@ inline Style::Length valueFromEdges(
     Style::Length edge,
     Style::Length axis,
     Style::Length defaultValue) {
-  if (edge.unit() != Unit::Undefined) {
+  if (!edge.isUndefined()) {
     return edge;
   }
-  if (axis.unit() != Unit::Undefined) {
+  if (!axis.isUndefined()) {
     return axis;
   }
   return defaultValue;