This feature aims to allow plugins that depend on the Purpur API to create custom NamespaceKey based enchantments.

Goals

There are a few goals that I want this PR to achieve before it gets merged:

[x] Allow plugins to add custom enchantments with the Bukkit API
[ ] Allow plugins to query and interact with custom enchantments without having to query new APIs (interop with old bukkit enchantments API)
[x] Allow players to apply custom enchantments with the vanilla enchant command (vanilla interop)
[ ] Make sure not to break other custom enchantment plugins

Considerations

Since this feature is supposed to be flexible for a lot of use-cases, there are some things that need to be considered:

[ ] The package where the code resides. Is org.bukkit.enchantments.custom even good enough? Should it be on a Purpur package?
[x] API structure. Should plugins have to implement a separate interface or extend from a Bukkit class? Should our CustomEnchantment class/interface extend/implement any Bukkit class/interface?
[ ] How to show it for clients. Show custom enchantments follow the same layout as regular enchantments? Should this change introduce a way to modify the layout of the enchantments as a configuration setting?
[ ] Anvil support. Does Vanilla even handle custom enchantments well with Anvils? What about books? Can we apply custom enchantments to items with an anvil? What about enchantment merging?
[ ] Exposing enchantments properly. Is this API flexible enough for anyone to use it? What about when enchantments need Listeners to function?