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-std/README.md

@@ -9,8 +9,8 @@ provide interactive utilities and building blocks for users writing casual scrip
 To see what's here:
 ```text
 > use std
-> scope commands | select name usage | where name =~ "std "
-#┬───────────name────────────┬──────────────────────usage──────────────────────
+> scope commands | select name description | where name =~ "std "
+#┬───────────name────────────┬───────────────────description───────────────────
 0│std assert                 │Universal assert command
 1│std assert equal           │Assert $left == $right
 2│std assert error           │Assert that executing the code generates an error