REPLCompletions: Add official hook for AI completions?

[Keno]

I think we should consider adding an official hook for Copilot-style AI completions in REPLCompletions.
It is of course possible to hack this in via monkey patching, but there are some non-trivial interactions with the rest of the REPLCompletions code regarding display priority and start/cancel (cf #52847), so I think it probably makes sense to have an explicit hook for it. I don't think we should ship with any enabled by default, but it should be easy to enabled (e.g. by loading the appropriate package in startup.jl).