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-lsp/src/lib.rs

@@ -353,11 +353,11 @@ impl LanguageServer {
                 let mut description = String::new();
 
                 // First description
-                description.push_str(&format!("{}\n", decl.usage().replace('\r', "")));
+                description.push_str(&format!("{}\n", decl.description().replace('\r', "")));
 
                 // Additional description
-                if !decl.extra_usage().is_empty() {
-                    description.push_str(&format!("\n{}\n", decl.extra_usage()));
+                if !decl.extra_description().is_empty() {
+                    description.push_str(&format!("\n{}\n", decl.extra_description()));
                 }
 
                 // Usage