fix(velocity): split onLoad and onEnable calls to late/early ProxyInitializeEvent handlers

[Kowagirl]

This splits the plugin's onLoad and onEnable between two separate ProxyInitializeEvent handlers.

onLoad is called on PostOrder.EARLY, with onEnable called on PostOrder.LATE.

This allows other plugins using the API to register variables on their initialization with the default order. At the moment, it is not possible to register variables cleanly as it will either error if done before BTLP is initialized, or BTLP will throw a warning when loading the tab list using the variable if done after. This fix replicates the order of operations that was done by registering variables in onLoad in plugins using this API.