This changeset contains everything that a separate binary needs to
register its own commands (including the new help function). It is
very possible that this commit misses other pub use exports, but
the contained ones work for our use cases so far.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
