fix(0.78): Add missing loadFromSource method in the DefaultRNFactoryDelegate (#2437)

## Summary:

Cherry pick
facebook@8b33668
to fix an issue where new apps made with the old architecture never call
metro and therefore show a blank screen.

## Test Plan:

Tested on a new app using `@react-native-community/cli init &&
react-native-macos-init`

Co-authored-by: Riccardo Cipolleschi <[email protected]>

Author: Saadnajmi

packages/react-native/Libraries/AppDelegate/RCTDefaultReactNativeFactoryDelegate.mm

@@ -122,4 +122,11 @@ - (Class)getModuleClassFromName:(const char *)name
   return nullptr;
 }
 
+- (void)loadSourceForBridge:(RCTBridge *)bridge
+                 onProgress:(RCTSourceLoadProgressBlock)onProgress
+                 onComplete:(RCTSourceLoadBlock)loadCallback
+{
+  [RCTJavaScriptLoader loadBundleAtURL:[self sourceURLForBridge:bridge] onProgress:onProgress onComplete:loadCallback];
+}
+
 @end

packages/rn-tester/RNTester/AppDelegate.mm

@@ -90,13 +90,6 @@ - (BOOL)application:(UIApplication *)app
 }
 #endif // [macOS]
 
-- (void)loadSourceForBridge:(RCTBridge *)bridge
-                 onProgress:(RCTSourceLoadProgressBlock)onProgress
-                 onComplete:(RCTSourceLoadBlock)loadCallback
-{
-  [RCTJavaScriptLoader loadBundleAtURL:[self sourceURLForBridge:bridge] onProgress:onProgress onComplete:loadCallback];
-}
-
 - (std::shared_ptr<facebook::react::TurboModule>)getTurboModule:(const std::string &)name
                                                       jsInvoker:(std::shared_ptr<facebook::react::CallInvoker>)jsInvoker
 {