Move _isShown() method to private section

[MohamadSalman11]

Description

I’m not sure if this is an oversight or on purpose, but I think the _isShown() method should be moved to the private section since it’s marked as private, especially since in js/src/dropdown.js it’s correctly placed under the private section. Please let me know if I missed anything. I’d appreciate your feedback!

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed