add ps and early help

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

@@ -1,7 +1,9 @@
 mod benchmark;
+mod ps;
 mod run_external;
 mod sys;
 
 pub use benchmark::Benchmark;
+pub use ps::Ps;
 pub use run_external::{External, ExternalCommand};
 pub use sys::Sys;