Add time states for PlayerItemHeldEvent and hotbar slot

[TheLimeGlass]

Description

• Adds time states for PlayerItemHeldEvent event.
• Adds time states for ExprHotbarSlot with PlayerItemHeldEvent event.
• Made the players in ExprHotbarSlot not required (default expression).
• Cleaned up ExprHotbarSlot class.

Test script:

on item held change:
	if the selected hotbar slot was a diamond:
		set the currently selected hotbar slot to slot 5 of player

	message "previous &c%past event-slot%"
	message "current &a%event-slot%"
	message "&6future %future event-slot%"

	# Works the same as above.
	message "previous &c%past selected hotbar slot of player%"
	message "current &a%currently selected hotbar slot of player%"
	message "&6future %future selected hotbar slot of player%"

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Target Minecraft Versions: any
Requirements: none
Related Issues: #5112