Add back [RCTCxxBridge reactInstance] method (facebook#647)

This method was removed for 0.63, but the `RCTBridge+Cxx.mm` category (which is marked OSS candidate) relies on it existing. This unblocks moving to 0.63, but we will consider upstreaming this to core.

React/CxxBridge/RCTCxxBridge.mm

@@ -220,6 +220,13 @@ - (void)setRCTTurboModuleLookupDelegate:(id<RCTTurboModuleLookupDelegate>)turboM
   return _jsMessageThread;
 }
 
+// [TODO(OSS Candidate ISS#2710739)
+- (std::weak_ptr<Instance>)reactInstance
+{
+  return _reactInstance;
+}
+// ]TODO(OSS Candidate ISS#2710739)
+
 - (BOOL)isInspectable
 {
   return _reactInstance ? _reactInstance->isInspectable() : NO;