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.

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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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[antonis]

Closing in favor of #4984 which was merged in v7