Add NexoHook for Nexo custom blocks and items

[Copilot]

BentoBox ships soft-dependency hooks for custom-item plugins (ItemsAdderHook, OraxenHook, CraftEngineHook) but had none for Nexo, the actively maintained Oraxen successor. This adds NexoHook, unblocking addon-agnostic custom-block generation (e.g. MagicCobblestoneGenerator).

Hook implementation

• New NexoHook mirroring CraftEngineHook's shape and soft-dependency pattern (hook() checks for the Nexo plugin, no hard compile-time requirement for addons).
• Static helpers delegate directly to Nexo's NexoBlocks / NexoItems API:

NexoHook.exists("mynamespace:my_block");           // NexoBlocks.isCustomBlock(id)
NexoHook.placeBlock(location, "mynamespace:my_block"); // NexoBlocks.place(blockId, location)
NexoHook.getBlockId(block);                        // NexoBlocks.customBlockMechanic(block).getItemID()
NexoHook.getItemStack("mynamespace:my_item");       // NexoItems.itemFromId(id).build()

Registration

• BentoBoxHookRegistrar.registerLateHooks() now registers NexoHook alongside the other late-loading custom-item-plugin hooks.

Build

• Added com.nexomc:nexo compileOnly dependency and the https://repo.nexomc.com/releases Maven repository to build.gradle.kts, following the existing Oraxen/CraftEngine dependency conventions.