feat(skeleton): add skeleton component in lib

[nouha06]

Note: Please transform - [ ] into - (NA) in the description when things are not applicable

Related issues

• In library, add component skeleton #867

Description

Motivation & Context

Types of change

• Bug fix (non-breaking 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)

Previews

Checklist

Contribution

• I have read the contributing guidelines

Accessibility

• My change follows accessibility good practices

Design

• My change respects the design guidelines of Orange Unified Design System

Development

• My change follows the developer guide
• I have added unit tests to cover my changes (optional)

Documentation

• My change introduces changes to the documentation and/or I have updated the documentation accordingly

Checklist (for Core Team only)

• Manually test (dark mode, RTL, landscape display, tablet)
• Documentation has been updated if relevant
• Design review
• A11y review
• Internal files have been updated if relevant (THIRD_PARTY, NOTICE)
• CHANGELOG.md files have been updated respecting keep a changelog rules and referencing the issue

[github-actions]

📖 Documentation preview deployed!

🔗 https://flutter.unified-design-system.orange.com/prPreview/969/