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/tests/commands/each.rs

@@ -11,3 +11,39 @@ fn each_works_separately() {
 
     assert_eq!(actual.out, "[11,12,13]");
 }
+
+#[test]
+fn each_group_works() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        echo [1 2 3 4 5 6] | each group 3 { echo $it } | to json
+        "#
+    ));
+
+    assert_eq!(actual.out, "[[1,2,3],[4,5,6]]");
+}
+
+#[test]
+fn each_window() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        echo [1 2 3 4] | each window 3 { echo $it } | to json
+        "#
+    ));
+
+    assert_eq!(actual.out, "[[1,2,3],[2,3,4]]");
+}
+
+#[test]
+fn each_window_stride() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        echo [1 2 3 4 5 6] | each window 3 -s 2 { echo $it } | to json
+        "#
+    ));
+
+    assert_eq!(actual.out, "[[1,2,3],[3,4,5]]");
+}