Publish top-level Flow types for `react-native` (#51908)

Summary:
FIXED Add index.js.flow to npm package files for Flow support

Currently, the distributed npm package for react-native does not include the index.js.flow file, which causes all exports to be typed as any when using Flow. This commit adds index.js.flow to the "files" array in package.json, ensuring Flow users receive proper type definitions out of the box. This addresses issues where type checking with Flow fails in React Native projects.

## Changelog:

[General][Added] Publish top-level Flow types for `react-native`

Pull Request resolved: https://github.com/facebook/react-native/pull/51908

Reviewed By: huntie, necolas

Differential Revision: D76292301

Pulled By: robhogan

fbshipit-source-id: e56360d3f35af30ef160470181349aac1812e7c1