Implement JustBetter_Core module

[claudio-ferraro]

Summary

This PR integrates the justbetter/magento2-core package into the Sentry module.

• The module configuration has been moved to the JustBetter tab in Stores > Configuration.
• The menu item is now nested under the main JustBetter menu.
• composer.json has been updated to require justbetter/magento2-core.

This change ensures consistency across JustBetter modules and centralizes the configuration access.

Result

• Configuration: Verified that the Sentry configuration is now accessible at Stores > Configuration > JustBetter > Sentry and loads without errors.
• Menu: Verified that the "Sentry" link appears correctly under the "JustBetter" menu in the admin panel.
• Code Quality: Ran static analysis and code style checks successfully.

Checklist

• I've ran composer run codestyle
• I've ran composer run analyse