fix(ios): persist position during page list head changes

[alpha0010]

Summary

Dynamic changes to the page list prior to the currently visible page now maintain the same visible page on iOS (this already works on Android).

Fixes #965

Test Plan

What's required for testing (prerequisites)?

function DemoPager() {
  const allPages = [0, 1, 2, 3, 4];
  const [pages, setPages] = useState(allPages);

  const togglePage = (page: number) =>
    setPages((prev) => {
      const itemIndex = prev.indexOf(page);
      const next = [...prev];
      if (itemIndex === -1) {
        next.push(page);
        next.sort((a, b) => a - b);
      } else {
        next.splice(itemIndex, 1);
      }
      return next;
    });

  return (
    <View style={styles.container}>
      <PagerView style={styles.container}>
        {pages.map((page) => (
          <View collapsable={false} key={`x:${page}`}>
            <Text>{page}</Text>
          </View>
        ))}
      </PagerView>
      <View style={styles.container}>
        {allPages.map((page) => (
          <Button
            key={`x:${page}`}
            onPress={() => togglePage(page)}
            title={`Toggle ${page}`}
          />
        ))}
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {flex: 1},
});

What are the steps to reproduce (after prerequisites)?

Use the above test screen to add/remove pages before/after the currently visible page. With this PR, iOS should now match the existing Android behavior.

Compatibility

OS	Implemented
iOS	✅
Android	N/A

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I updated the typed files (TS and Flow)