refactor: improve code structure and readability for CoreValidator

[pryzmian]

• Moved ConfigurationOptions, EnvironmentVariables, and PackageJson types to ICoreValidator.ts for better organization.
• Renamed methods for enhanced clarity and consistency.
• Replaced non-null assertions with default values for safer code.
• Refined environment variable validation and error handling.
• Simplified YouTube authentication token validation.
• Updated comments and logging for better maintainability.
• Adapted logic for unit tests