Allow external binary to register custom commands. (#1780)

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.

crates/nu-cli/src/commands/help.rs

@@ -131,7 +131,7 @@ You can also learn more at https://www.nushell.sh/book/"#;
 }
 
 #[allow(clippy::cognitive_complexity)]
-pub(crate) fn get_help(
+pub fn get_help(
     cmd: &dyn WholeStreamCommand,
     registry: &CommandRegistry,
 ) -> impl Into<OutputStream> {