feat(iOS): expose RCT_NEW_ARCH_ENABLED to Swift (#46749)

Summary:
This PR exposes `RCT_NEW_ARCH_ENABLED` flag to Swift.

It allows you to use conditional compilation:

```swift
#if RCT_NEW_ARCH_ENABLED
func test() {
	print("I'm running on new arch!)
}
#else
func test() {
	print("I'm running on old arch!)
}
#endif
```

## Changelog:

[IOS] [ADDED] - expose RCT_NEW_ARCH_ENABLED to Swift

Pull Request resolved: #46749

Test Plan: CI Green

Reviewed By: cortinico

Differential Revision: D63689711

Pulled By: cipolleschi

fbshipit-source-id: 706013019571f597d1966ffcf13bd47ba7f29a2d

packages/react-native/scripts/cocoapods/new_architecture.rb

@@ -99,6 +99,7 @@ def self.install_modules_dependencies(spec, new_arch_enabled, folly_version = ge
 
 
         ReactNativePodsUtils.add_flag_to_map_with_inheritance(current_config, "OTHER_CPLUSPLUSFLAGS", self.computeFlags(new_arch_enabled))
+        ReactNativePodsUtils.add_flag_to_map_with_inheritance(current_config, "OTHER_SWIFT_FLAGS", new_arch_enabled ? " -DRCT_NEW_ARCH_ENABLED" : "")
 
         spec.dependency "React-RCTFabric" # This is for Fabric Component
         spec.dependency "ReactCodegen"