feat: related #344 - handle items count and loader btn behaviors internally

[mattgoud]

❓ Types of changes

• handle items count internally (remove prop itemCount and retrieve items count from a computed function instead )
• handle disabled Load More btn internally (remove loaderButtonDisabled prop )

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• 📦 New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

📝 Checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes
• The component exists on old Prestashop UIKit and my pull request on migrating documentation is accepted.