Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

crates/nu-cli/tests/commands/nth.rs

@@ -0,0 +1,39 @@
+#[test]
+fn selects_a_row() {
+    Playground::setup("nth_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("notes.txt"), EmptyFile("arepas.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | sort-by name
+                | nth 0
+                | get name
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "arepas.txt");
+    });
+}
+
+#[test]
+fn selects_many_rows() {
+    Playground::setup("nth_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("notes.txt"), EmptyFile("arepas.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | get name
+                | nth 1 0
+                | count
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "2");
+    });
+}