Add `NodeInjectionService` (#153, solution 2)

[VelvetToroyashi]

This PR implements support for solution 2 mentioned in #153.

I've yet to add tests (should be fairly simple). Solution 1 may have been easier considering interactivity being rewritten to piggyback off of Remora.Commands, but I already had the code written, and hadn't pushed.

If solution one is preferred, 90% of the code can be reused so feel free to poke about that. cc @Nihlus

( Fixes #153 )