Sleep command (#2381)

* Add deserialization of Primitive::Duration; Fixes #2373

* Implement Sleep command

* Add comment saying you should name your rest field "rest"

* Fix typo

* Add documentation for sleep command

crates/nu-parser/src/parse.rs

@@ -334,7 +334,7 @@ fn parse_unit(lite_arg: &Spanned<String>) -> (SpannedExpression, Option<ParseErr
                     lhs.pop();
                 }
 
-                // these units are allowed to signed
+                // these units are allowed to be signed
                 if let Ok(x) = lhs.parse::<i64>() {
                     let lhs_span =
                         Span::new(lite_arg.span.start(), lite_arg.span.start() + lhs.len());