Implement Slash Commands framework

[JakenVeina]

I'm thinking MODiX 2.0 should support ONLY slash commands, so implement the infrastructure needed for this.

This should include a variety of default behaviors for reporting command errors, checking command permissions, reporting permission errors, reporting parameter mapping errors, etc. Some of this may dip into the Remora framework, in which case we should work with Jax to make whatever enhancements are needed.

Go ahead and implement diagnostics commands, under this scope, as verification of the infrastructure.