Multi line text view key handling: add check for first responder

performKeyEquivalent is [documented](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/EventOverview/EventArchitecture/EventArchitecture.html#//apple_ref/doc/uid/10000060i-CH3-SW10) to be called even if a view isn't first responder (this is for example how "Enter" binds to the default button on a dialog, for example -- the default button doesn't need to be focused to respond to Enter), but since we override performKeyEquivalent to properly support certain keyboard shortcuts (like noted in #1867), the previous assumption on the actual use of performKeyEquivalent no longer holds -- it's *only* for keystrokes sent in focused state, so add that check.

packages/react-native/Libraries/Text/TextInput/Multiline/RCTUITextView.mm

@@ -577,7 +577,7 @@ - (void)deleteBackward {
 }
 #else // [macOS
 - (BOOL)performKeyEquivalent:(NSEvent *)event {
-  if (!self.hasMarkedText && ![self.textInputDelegate textInputShouldHandleKeyEvent:event]) {
+  if (self.window.firstResponder == self && !self.hasMarkedText && ![self.textInputDelegate textInputShouldHandleKeyEvent:event]) {
     return YES;
   }