Add search terms to Command and Signature (#4980)

* Add search terms to command

* Rename Signature desc to usage

To be named uniformly with extra_usage

* Throw in foldl search term for reduce

* Add missing usage to post

* Add search terms to signature

* Try to add capnp Signature serialization

crates/nu-command/src/experimental/view_source.rs

@@ -20,7 +20,6 @@ impl Command for ViewSource {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("view-source")
-            .desc(self.usage())
             .required("item", SyntaxShape::Any, "name or block to view")
             .category(Category::Core)
     }