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-cli/src/commands/commandline/commandline_.rs

@@ -14,7 +14,7 @@ impl Command for Commandline {
             .category(Category::Core)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "View the current command line input buffer."
     }
 

crates/nu-cli/src/commands/commandline/edit.rs

@@ -34,7 +34,7 @@ impl Command for SubCommand {
             .category(Category::Core)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Modify the current command line input buffer."
     }
 

crates/nu-cli/src/commands/commandline/get_cursor.rs

@@ -16,7 +16,7 @@ impl Command for SubCommand {
             .category(Category::Core)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the current cursor position."
     }
 

crates/nu-cli/src/commands/commandline/set_cursor.rs

@@ -22,7 +22,7 @@ impl Command for SubCommand {
             .category(Category::Core)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Set the current cursor position."
     }
 

crates/nu-cli/src/commands/history/history_.rs

@@ -13,7 +13,7 @@ impl Command for History {
         "history"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the command history."
     }
 

crates/nu-cli/src/commands/history/history_session.rs

@@ -8,7 +8,7 @@ impl Command for HistorySession {
         "history session"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get the command history session."
     }
 

crates/nu-cli/src/commands/keybindings.rs

@@ -14,11 +14,11 @@ impl Command for Keybindings {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Keybindings related commands."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         r#"You must use one of the following subcommands. Using this command as-is will only produce this help message.
 
 For more information on input and keybindings, check:

crates/nu-cli/src/commands/keybindings_default.rs

@@ -15,7 +15,7 @@ impl Command for KeybindingsDefault {
             .input_output_types(vec![(Type::Nothing, Type::table())])
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "List default keybindings."
     }
 

crates/nu-cli/src/commands/keybindings_list.rs

@@ -23,7 +23,7 @@ impl Command for KeybindingsList {
             .category(Category::Platform)
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "List available options that can be used to create keybindings."
     }
 

crates/nu-cli/src/commands/keybindings_listen.rs

@@ -12,11 +12,11 @@ impl Command for KeybindingsListen {
         "keybindings listen"
     }
 
-    fn usage(&self) -> &str {
+    fn description(&self) -> &str {
         "Get input from the user."
     }
 
-    fn extra_usage(&self) -> &str {
+    fn extra_description(&self) -> &str {
         "This is an internal debugging tool. For better output, try `input listen --types [key]`"
     }