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/reverse.rs

@@ -0,0 +1,11 @@
+use nu_test_support::nu;
+
+#[test]
+fn can_get_reverse_first() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats",
+        "ls | sort-by name | reverse | first 1 | get name | trim | echo $it"
+    );
+
+    assert_eq!(actual, "utf16.ini");
+}