feat(bottom-tabs,iOS): throw error on more than single focused tab in debug mode (#16)

## Description

Added assertion, verifying that only a single tab is focused.
The assertion is currently enabled only in debug mode.

## Test code and steps to reproduce

Pass `isFocused={true}` to two tabs.

## Checklist

- [ ] Ensured that CI passes

Author: kkafar

ios/bottom-tabs/RNSTabBarController.mm

@@ -88,6 +88,10 @@ - (void)updateSelectedViewController
 {
   _needsUpdateOfSelectedTab = false;
 
+#if !defined(NDEBUG)
+  [self assertExactlyOneFocusedTab];
+#endif
+
   UIViewController *_Nullable selectedViewController = nil;
   for (RNSTabsScreenViewController *tabViewController in self.viewControllers) {
     NSLog(
@@ -132,4 +136,18 @@ - (void)updateTabBarAppearance
                                     tabScreenControllers:_tabScreenControllers];
 }
 
+#if !defined(NDEBUG)
+- (void)assertExactlyOneFocusedTab
+{
+  int selectedCount = 0;
+  for (RNSTabsScreenViewController *tabViewController in _tabScreenControllers) {
+    if (tabViewController.tabScreenComponentView.isFocused) {
+      ++selectedCount;
+    }
+  }
+  RCTAssert(
+      selectedCount == 1, @"[RNScreens] Invariant violation. Expected exactly 1 focused tab, got: %d", selectedCount);
+}
+#endif
+
 @end