Add which command, add external completions, and builtin var completions (#782)

* Add which and external completions

* WIP

* Finish up external and var completions

* fix windows

crates/nu-command/src/system/mod.rs

@@ -2,8 +2,10 @@ mod benchmark;
 mod ps;
 mod run_external;
 mod sys;
+mod which_;
 
 pub use benchmark::Benchmark;
 pub use ps::Ps;
 pub use run_external::{External, ExternalCommand};
 pub use sys::Sys;
+pub use which_::Which;