Add missing strictMode prop to FlatList TypeScript types (#57674)

Summary:
FlatList's `strictMode` prop is implemented in the component and declared in the Flow types (OptionalFlatListProps in FlatList.js), but missing from the shipped TypeScript definitions. TS users can't set it on <FlatList> without augmenting the module themselves. This adds `strictMode?: boolean` to the TypeScript FlatListProps, matching the Flow declaration. Types-only, no runtime change.

## Changelog:

[GENERAL] [ADDED] - Add missing strictMode prop to FlatList TypeScript types

Pull Request resolved: #57674

Test Plan:
Types-only change:
- Before, `<FlatList ... strictMode />` errored `(Property 'strictMode' does not exist);`
- After, it type-checks. Matches the Flow prop and the runtime use in `render(): const renderer = strictMode ? this._memoizedRenderer : this._renderer;`

Reviewed By: cortinico

Differential Revision: D113631325

Pulled By: Abbondanzo

fbshipit-source-id: 84866649daec3385b564e4c13f139aea0a787b37

Author: drhops

packages/react-native/Libraries/Lists/FlatList.d.ts

@@ -161,6 +161,11 @@ export interface FlatListProps<ItemT> extends VirtualizedListProps<ItemT> {
    * @platform android
    */
   fadingEdgeLength?: number | {start: number; end: number} | undefined;
+
+  /**
+   * Enable an optimization to memoize the item renderer to prevent unnecessary rerenders.
+   */
+  strictMode?: boolean | undefined;
 }
 
 export abstract class FlatListComponent<