Change the usage misnomer to "description" (#13598)

# Description
    
The meaning of the word usage is specific to describing how a command
function is *used* and not a synonym for general description. Usage can
be used to describe the SYNOPSIS or EXAMPLES sections of a man page
where the permitted argument combinations are shown or example *uses*
are given.
Let's not confuse people and call it what it is a description.

Our `help` command already creates its own *Usage* section based on the
available arguments and doesn't refer to the description with usage.

# User-Facing Changes

`help commands` and `scope commands` will now use `description` or
`extra_description`
`usage`-> `description`
`extra_usage` -> `extra_description`

Breaking change in the plugin protocol:

In the signature record communicated with the engine.
`usage`-> `description`
`extra_usage` -> `extra_description`

The same rename also takes place for the methods on
`SimplePluginCommand` and `PluginCommand`

# Tests + Formatting
- Updated plugin protocol specific changes
# After Submitting
- [ ] update plugin protocol doc

crates/nu-protocol/tests/test_signature.rs

@@ -9,14 +9,14 @@ fn test_signature() {
     assert_eq!(signature, from_build);
 
     // constructing signature with description
-    let signature = Signature::new("signature").usage("example usage");
-    assert_eq!(signature.usage, "example usage".to_string())
+    let signature = Signature::new("signature").description("example description");
+    assert_eq!(signature.description, "example description".to_string())
 }
 
 #[test]
 fn test_signature_chained() {
     let signature = Signature::new("new_signature")
-        .usage("description")
+        .description("description")
         .required("required", SyntaxShape::String, "required description")
         .optional("optional", SyntaxShape::String, "optional description")
         .required_named(
@@ -126,7 +126,7 @@ fn test_signature_same_name() {
 #[test]
 fn test_signature_round_trip() {
     let signature = Signature::new("new_signature")
-        .usage("description")
+        .description("description")
         .required("first", SyntaxShape::String, "first required")
         .required("second", SyntaxShape::Int, "second required")
         .optional("optional", SyntaxShape::String, "optional description")
@@ -145,8 +145,8 @@ fn test_signature_round_trip() {
     let returned: Signature = serde_json::from_str(&string).unwrap();
 
     assert_eq!(signature.name, returned.name);
-    assert_eq!(signature.usage, returned.usage);
-    assert_eq!(signature.extra_usage, returned.extra_usage);
+    assert_eq!(signature.description, returned.description);
+    assert_eq!(signature.extra_description, returned.extra_description);
     assert_eq!(signature.is_filter, returned.is_filter);
     assert_eq!(signature.category, returned.category);