Create definitions of feature flags for React in internal module within the react-native package

[rubennorte]

Summary: Changelog: [internal]

This creates a new module in react-native (specifically react-native/src/private/renderer/featureflags/ReactFeatureFlags.js) that will replace the arbitrary ReactNativeInternalFeatureFlags module that React is consuming to customize behaviors for RN.

This will allow us to consume feature flags defined in the canonical feature flag system that we have in React Native (ReactNativeFeatureFlags.js).

The order in which PRs should land is:

1. This change, which adds the definitions for the new module in RN and the feature flags that they're mapped to.
2. The change in the React repository to start consuming this new module.
3. A clean up for the legacy module.

Differential Revision: D62762838

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D62762838

[react-native-bot]

This PR is stale because it has been open for 180 days with no activity. It will be closed in 7 days unless you comment on it or remove the "Stale" label.