Sort entries in scope commands; Fix usage of externs (#10039)

# Description

* All output of `scope` commands is sorted by the "name" column. (`scope
externs` and some other commands had entries in a weird/random order)
* The output of `scope externs` does not have extra newlines (that was
due to wrong usage creation of known externals)

crates/nu-parser/src/known_external.rs

@@ -11,6 +11,7 @@ pub struct KnownExternal {
     pub name: String,
     pub signature: Box<Signature>,
     pub usage: String,
+    pub extra_usage: String,
 }
 
 impl Command for KnownExternal {