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_match/src/nu/mod.rs

@@ -10,7 +10,7 @@ use regex::Regex;
 impl Plugin for Match {
     fn config(&mut self) -> Result<Signature, ShellError> {
         Ok(Signature::build("match")
-            .desc("Filter rows by Regex pattern")
+            .desc("Filter rows by Regex pattern.")
             .required("member", SyntaxShape::String, "the column name to match")
             .required("regex", SyntaxShape::String, "the regex to match with")
             .switch("insensitive", "case-insensitive search", Some('i'))