Fix: Implement a fix for displaying non-freezed placeholders internally or with PAPI

[Klema4]

This pull request introduces improvements to user loading logic, enhances PlaceholderAPI support for experience-related placeholders, and refines leaderboard status handling. The changes aim to make user state transitions more robust, provide more flexible placeholder options, and improve clarity in leaderboard display.

User loading and state management:

• Refactored the user loading process in UserManagerImpl.java to ensure that offline users are properly converted to online users using the new toOnlineUser method, reducing code duplication and improving clarity. [1] [2]

PlaceholderAPI enhancements:

• Added support for additional experience-related placeholders (e.g., experience_minimum, player_experience, player_experience_required, etc.) in PlaceholderAPI.java, making the API more flexible and consistent for plugin users. [1] [2]

Leaderboard display improvements:

• Updated leaderboard placeholder logic to show a loading name when the leaderboard is updating, rather than always showing a no-player name, improving user feedback during leaderboard updates.

Listener registration cleanup:

• Removed a redundant call to register() within the piston retract event handler in Listeners.java, preventing unnecessary registration logic.…; enhance PlaceholderAPI with additional experience-related identifiers; remove unused register call in Listeners.