Add ability to pass ItemSeparatorComponent as React Element (#32748)

Summary:
Currently `ListHeaderComponent` & `ListFooterComponent` allow to use React Componetn & Elemelen

```tsx
<FlatList
  ListHeaderComponent={<View />} // valid
  ListHeaderComponent={View}     // valid
/>
```

But when you try to pass `ItemSeparatorComponent` as React Element it will throw an error

```tsx
<FlatList
  ItemSeparatorComponent={View}     // ok
  ItemSeparatorComponent={<View />} /* not valid:
    Error: Element type is invalid: expected a string (for built-in components) or a class/function
    (for composite components) but got: object.
    Check the render method of `CellRenderer`.
  */
/>
```

So, this PR adds this ability

## Changelog

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[General] [Changed] - Add ability to pass `ItemSeparatorComponent` as React Element

Pull Request resolved: #32748

Test Plan: ...

Reviewed By: lyahdav

Differential Revision: D37227719

Pulled By: cortinico

fbshipit-source-id: 1c4943fa9d42bf5e61fbd999d1f5be46b51ecb14

Libraries/Lists/VirtualizedList.js

@@ -2097,9 +2097,11 @@ class CellRenderer extends React.Component<
         : this._onLayout;
     // NOTE: that when this is a sticky header, `onLayout` will get automatically extracted and
     // called explicitly by `ScrollViewStickyHeader`.
-    const itemSeparator = ItemSeparatorComponent && (
-      <ItemSeparatorComponent {...this.state.separatorProps} />
-    );
+    const itemSeparator = React.isValidElement(ItemSeparatorComponent)
+      ? ItemSeparatorComponent
+      : ItemSeparatorComponent && (
+          <ItemSeparatorComponent {...this.state.separatorProps} />
+        );
     const cellStyle = inversionStyle
       ? horizontal
         ? [styles.rowReverse, inversionStyle]