Add an onItemEquipped event to Bedrock Script API

Currently, there is no native event in the Script API to detect when a player equips an item (main hand/armor). ### Disadvantages of not having this event: * Performance Lag: Developers must use `system.runInterval` every tick to scan players' inventories, causing heavy CPU overhead on servers/Realms. * Code Complexity: Requires writing complex, inefficient caching systems just to compare previous and current item IDs. * Inaccuracy: Swapping between two identical items with different durability/enchantments is difficult to detect via standard tick loops. ### Proposed Solution: Add a built-in event like `world.afterEvents.playerEquipItem` that triggers instantly when a slot content changes, improving both performance and add-on capabilities.