Add "old" implementation for reference & comparison

ios/RNSScreenStack.mm

@@ -750,7 +750,11 @@ - (void)dismissOnReload
       // otherwise the screen will be just popped immediately due to no animation
       ((operation == UINavigationControllerOperationPop && shouldCancelDismiss) || _isFullWidthSwiping ||
        [RNSScreenStackAnimator isCustomAnimation:screen.stackAnimation] || _customAnimation)) {
-    return [[RNSScreenStackAnimator alloc] initWithOperation:operation];
+    if (rns::kUsesNewAnimatorImpl) {
+      return [[RNSScreenStackAnimator alloc] initWithOperation:operation];
+    } else {
+      return [[RNSScreenStackAnimatorLegacy alloc] initWithOperation:operation];
+    }
   }
   return nil;
 }

ios/RNSScreenStackAnimator.h

@@ -1,5 +1,11 @@
+#pragma once
+
 #import "RNSScreen.h"
 
+namespace rns {
+constexpr bool kUsesNewAnimatorImpl = false;
+}
+
 @interface RNSScreenStackAnimator : NSObject <UIViewControllerAnimatedTransitioning>
 
 /// This property is filled whenever there is an ongoing animation and cleared on animation end.
@@ -18,3 +24,22 @@
 + (BOOL)isCustomAnimation:(RNSScreenStackAnimation)animation;
 
 @end
+
+@interface RNSScreenStackAnimatorLegacy : NSObject <UIViewControllerAnimatedTransitioning>
+
+/// This property is filled whenever there is an ongoing animation and cleared on animation end.
+@property (nonatomic, strong, nullable, readonly) UIViewPropertyAnimator *inFlightAnimator;
+
+- (nonnull instancetype)initWithOperation:(UINavigationControllerOperation)operation;
+
+/// In case of interactive / interruptible transition (e.g. swipe back gesture) this method should return
+/// timing parameters expected by animator to be used for animation completion (e.g. when user's
+/// gesture had ended).
+///
+/// @return timing curve provider expected to be used for animation completion or nil,
+/// when there is no interactive transition running.
+- (nullable id<UITimingCurveProvider>)timingParamsForAnimationCompletion;
+
++ (BOOL)isCustomAnimation:(RNSScreenStackAnimation)animation;
+
+@end