each group and each window subcommands. (#2508)

* First commit updating `config` to use subcommands (#2119)
    - Implemented `get` subcommand

* Implmented `config set` as a subcommand.

* Implemented `config set_into` as subcommand

* Fixed base `config` command
 - Instead of outputting help, it now outputs the list of all
 configuration parameters.

* Added `config clear` subcommand

* Added `config load` and `config remove` subcommands

* Added `config path` subcommand

* fixed clippy

* initial commit for implementing groups

* each group works

* each group is slightly cleaner + added example

* Added `each window` subcommand
    - No support for stride flag yet

* each window stride implemented

* Added tests and minor documentation changes

* fixed clippy

* fixed clippy again

crates/nu-cli/src/cli.rs

@@ -205,6 +205,8 @@ pub fn create_default_context(
             whole_stream_command(Rename),
             whole_stream_command(Uniq),
             whole_stream_command(Each),
+            whole_stream_command(EachGroup),
+            whole_stream_command(EachWindow),
             whole_stream_command(IsEmpty),
             // Table manipulation
             whole_stream_command(Move),