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 {

crates/nu-parser/src/parse_keywords.rs

@@ -215,6 +215,7 @@ pub fn parse_def_predecl(working_set: &mut StateWorkingSet, spans: &[Span]) {
             let decl = KnownExternal {
                 name,
                 usage: "run external command".into(),
+                extra_usage: "".into(),
                 signature,
             };
 
@@ -650,7 +651,8 @@ pub fn parse_extern(
                 } else {
                     let decl = KnownExternal {
                         name: external_name,
-                        usage: [usage, extra_usage].join("\n"),
+                        usage,
+                        extra_usage,
                         signature,
                     };