This adds the ability for plugins to define completion handlers as async safe (not on games main thread)
When they are defined async safe, and ran on a Paper 1.12.2+ server, with a Paper ACF manager,
completions will be handled mostly async, letting you safely do heavier operations in tab completions.
For consistency, make Completion Handlers be implemented the same way Context Handlers are.
Previously when we added the Context parameter, we left the original signature in place and simply tacked , c on the end.
This made migration easier than changing all completion handlers bodies.
But since 0.5.0 is a big migration already, let's just get it over with.
See changes to Bukkit default completions and the example plugin for migration example.
Aikar