Allow composing help message from two parts (#3124)

* Split help message into brief and full help

Demonstrate on ansi command

Brief help is printed when running `help commands` so it doesn't clutter
the table. Full help is printed when normal help message is requested
(e.g., `help ansi`, `ansi --help`, etc.).

* Split long command descriptions

Some are not split, just edited to be shorter.

* Capitalize the usage of all commands

* Make sure every usage ends with dot

* Fix random typo

crates/nu-command/src/commands/keep/command.rs

@@ -26,7 +26,7 @@ impl WholeStreamCommand for Command {
     }
 
     fn usage(&self) -> &str {
-        "Keep the number of rows only"
+        "Keep the number of rows only."
     }
 
     async fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {