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/old/nu_plugin_to_bson/src/nu/mod.rs

@@ -10,7 +10,7 @@ use nu_source::Tag;
 impl Plugin for ToBson {
     fn config(&mut self) -> Result<Signature, ShellError> {
         Ok(Signature::build("to bson")
-            .desc("Convert table into .bson binary")
+            .usage("Convert table into .bson binary")
             .filter())
     }