Add assembly discovery extension methods

[VelvetToroyashi]

This PR adds helper methods that register responders and commands based on assembly discovery.

This was pitched by @cmsteffey, since other libraries have this functionality (usually in addition to explicit registrations, such as D#+)

[Nihlus]

Still waiting for tests - a rebase to avoid CI errors might not be a bad idea either.

[VelvetToroyashi]

Right. Tests. I'll get to this this afternoon.

[VelvetToroyashi]

This kids is why you don't rebase.

[VelvetToroyashi]

Tests have been added and pass