Begin directory contrib docs and split commands (#3650)

* Begin directory contrib docs and split commands

* Fix unused import warning

crates/nu-engine/src/lib.rs

@@ -13,6 +13,7 @@ pub mod plugin;
 mod print;
 pub mod script;
 pub mod shell;
+mod types;
 mod whole_stream_command;
 
 pub use crate::call_info::UnevaluatedCallInfo;

crates/nu-engine/src/types.rs

@@ -0,0 +1 @@
+pub(crate) mod deduction;