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/edit.rs

@@ -0,0 +1,16 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn creates_a_new_table_with_the_new_row_given() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open cargo_sample.toml
+            | edit dev-dependencies.pretty_assertions "7"
+            | get dev-dependencies.pretty_assertions
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "7");
+}