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/append.rs

@@ -0,0 +1,30 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn adds_a_row_to_the_end() {
+    Playground::setup("append_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "los_tres_caballeros.txt",
+            r#"
+                Andrés N. Robalino
+                Jonathan Turner
+                Yehuda Katz
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open los_tres_caballeros.txt
+                | lines
+                | append "pollo loco"
+                | nth 3
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "pollo loco");
+    })
+}