nu-cli refactor moving commands into their own crate nu-command (#2910)

* move commands, futures.rs, script.rs, utils

* move over maybe_print_errors

* add nu_command crate references to nu_cli

* in commands.rs open up to pub mod from pub(crate)

* nu-cli, nu-command, and nu tests are now passing

* cargo fmt

* clean up nu-cli/src/prelude.rs

* code cleanup

* for some reason lex.rs was not formatted, may be causing my error

* remove mod completion from lib.rs which was not being used along with quickcheck macros

* add in allow unused imports

* comment out one failing external test; comment out one failing internal test

* revert commenting out failing tests; something else might be going on; someone with a windows machine should check and see what is going on with these failing windows tests

* Update Cargo.toml

Extend the optional features to nu-command

Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>

crates/nu-command/tests/commands/split_by.rs

@@ -0,0 +1,54 @@
+use nu_test_support::fs::Stub::{EmptyFile, FileWithContentToBeTrimmed};
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn splits() {
+    Playground::setup("split_by_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "los_tres_caballeros.csv",
+            r#"
+                first_name,last_name,rusty_at,type
+                Andrés,Robalino,10/11/2013,A
+                Jonathan,Turner,10/12/2013,B
+                Yehuda,Katz,10/11/2013,A
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open los_tres_caballeros.csv
+                | group-by rusty_at
+                | split-by type
+                | get A."10/11/2013"
+                | count
+            "#
+        ));
+
+        assert_eq!(actual.out, "2");
+    })
+}
+
+#[test]
+fn errors_if_no_table_given_as_input() {
+    Playground::setup("split_by_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![
+            EmptyFile("los.txt"),
+            EmptyFile("tres.txt"),
+            EmptyFile("amigos.txt"),
+            EmptyFile("arepas.clu"),
+        ]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | get name
+                | split-by type
+            "#
+        ));
+
+        assert!(actual.err.contains("Expected table from pipeline"));
+    })
+}