Mock REAInitializer for backward compatibility (#4880)

## Summary

Fixes
#4631

In #4619
I removed REAInitializer because it is no longer needed. However, some
brownfield apps still use it. This PR adds a mock for REAInitializer to
ensure backward compatibility.

## Test plan

CI

ios/native/REAInitializer.h

@@ -18,6 +18,10 @@ NS_ASSUME_NONNULL_BEGIN
 
 namespace reanimated {
 
+[[deprecated(
+    "REAInitializer method is no longer required, you can just remove invocation.")]] void
+REAInitializer(RCTBridge *bridge);
+
 #if REACT_NATIVE_MINOR_VERSION <= 71
 [[deprecated(
     "REAJSIExecutorRuntimeInstaller method is no longer required, you can just remove invocation.")]] JSIExecutor::

ios/native/REAInitializer.mm

@@ -4,6 +4,11 @@
 
 namespace reanimated {
 
+void REAInitializer(RCTBridge *bridge)
+{
+  // do nothing, just for backward compatibility
+}
+
 #if REACT_NATIVE_MINOR_VERSION <= 71
 
 JSIExecutor::RuntimeInstaller REAJSIExecutorRuntimeInstaller(