Update and fix shopkeepers integration

[tal5]

Based on bug report on Discord.

Changes

• Bumped Shopkeepers dependency to 2.16.6.
• ShopKeeperTag#wrapTradingRecipe -> ShopkeepersBridge#tradingRecipeToList.
• Updated ShopKeeperCreatedScriptEvent to modern handling & meta.
• Removed listening to PlayerCreatePlayerShopkeeperEvent in ShopKeeperCreatedScriptEvent, as it already listens to PlayerCreateShopkeeperEvent which it extends.
• ShopKeeperCreatedScriptEvent#onShopKeeperCreated -> onPlayerCreateShopkeeper (convention, should match the event name).
• Updated ShopKeeperTradeScriptEvent to modern handling & meta.
• Removed redundant player field from ShopKeeperTradeScriptEvent and PlayerCreateShopkeeperEvent.
• Improved ShopKeeperTag#valueOf errors.
• Added a starts with prefix -> true check in ShopKeeperTag#matches.
• Simplified ShopKeeperTag#isShopKeeper - use ShopkeepersAPI instead of casting the bridge's plugin instance, and one-line the check.
• Minor improvement to ShopKeeperTag#fromEntity - Null check ShopkeeperRegistry#getShopkeeperByEntity instead of checking if it's a shopkeeper twice (ShopkeeperRegistry#isShopkeeper just checks if getShopkeeperByEntity isn't null).
• Fixed null handling in ShopKeeperTag#getDenizenEntity - it was treated as non-null, but ShopKeeperTag#getBukkitEntity can return null.
• Minor cleanup to ShopKeeperTag#getBukkitEntity - one-line it using instanceof pattern matching.
• Updated ShopKeeperTag to modern tag handling (ObjectTagProcessor + register method).
• Minor meta fixes in ShopKeeperTag, notably the return type in ShopKeeperTag.trades and ShopKeeperTag.entity being nullable.
• Implemented getJavaObject in ShopKeeperTag.
• Implemented debuggable in ShopKeeperTag.
• Slightly moved methods around in ShopKeeperTag to have less relevant methods like prefix stuff at the bottom.
• Updated ShopKeepersEntityProperties to a modern ShopKeepersEntityExtensions extension class.
• Fixed EntityTag.shopkeeper meta - it can return null if the entity isn't a shopkeeper, and added a reference to is_shopkeeper.
• Updated ShopKeeperTag.trades to use the ListTag convertor constructor.
• It seems a shopkeeper's UI being active isn't a single set value anymore but managed per UI session - used a Stream#anyMatch to update ShopKeeperTag.is_ui_active for now, and added a TODO for better handling.
• Shopkeeper#isActive was removed in favor of ShopObject#isActive, so updated ShopKeeperTag.is_active for that.

Notes

• Do we want to deprecate EntityTag.is_shopkeeper in favor of EntityTag.shopkeeper + fallbacks?

[mcmonkey4eva]

Impressively thorough!

is_shopkeeper is a bit redundant, but doesn't cause any harm. There's quite a few redundant is_/has_ style tags that are retained.