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_plugin_fetch/src/nu/mod.rs

@@ -9,7 +9,7 @@ use crate::Fetch;
 impl Plugin for Fetch {
     fn config(&mut self) -> Result<Signature, ShellError> {
         Ok(Signature::build("fetch")
-            .desc("Load from a URL into a cell, convert to table if possible (avoid by appending '--raw')")
+            .desc("Load from a URL into a cell, convert to table if possible (avoid by appending '--raw').")
             .required(
                 "URL",
                 SyntaxShape::String,