feat: add minimum hearts threshold for heart item spawning

[Scralar]

What this does

Added a minimum hearts threshold system that prevents players from dropping or giving heart items when they die if they have below a certain number of hearts. Players still lose hearts from their max health, but no heart items are spawned.

New stuff

• Config option: minHeartsToSpawnHeartItem - controls minimum hearts required to spawn heart items
• Event properties: heartSpawningBlocked in death events for plugin integration
• Comprehensive coverage - applies to all heart dropping mechanisms (PvP, natural, cooldown, max hearts)
• Backward compatible - disabled by default (set to 0)

How it works

When a player dies with hearts below the threshold:

• They still lose hearts from their max health
• No heart items are dropped on the ground
• Killer doesn't receive heart items from them
• Works for both PvP and natural deaths
• Applies to all heart dropping scenarios (cooldown, max hearts, etc.)

Files changed

• Added minHeartsToSpawnHeartItem config option with documentation
• Added canSpawnHeartItem() utility method in PlayerDeathListener
• Updated handlePvPDeath() and handleNaturalDeath() to check threshold
• Fixed cooldown and max hearts drop logic to respect threshold
• Added heartSpawningBlocked property to death events
• Updated README with new config option

Testing

Built and compiled fine, follows existing code patterns. All heart dropping mechanisms now respect the threshold.

Notes

• Default value is 0 (disabled) to maintain backward compatibility
• Players below threshold still lose hearts but don't spawn items
• No breaking changes
• README and config documentation updated