feat(featureFlags): Re-export Feature Flags integration

[krystofwoldrich]

The packages/core/src/js/index.ts file was modified to export featureFlagsIntegration and type FeatureFlagsIntegration from @sentry/core.

• The FeatureFlagsIntegration type was added to the existing type exports block.
• The featureFlagsIntegration function was added to the existing function exports block.

This change enables the featureFlagsIntegration and its associated type to be re-exported from @sentry/core, making them accessible for use in other Sentry SDKs, such as @sentry/react-native. The modification aligns with the established pattern for exporting integrations within the Sentry codebase. A pull request was prepared to integrate these changes.

[github-actions]

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🚫	Please consider adding a changelog entry for the next release.

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📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Re-export Feature Flags integration ([#4917](https://github.com/getsentry/sentry-react-native/pull/4917))

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