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/split_column.rs

@@ -0,0 +1,20 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn by_column() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open cargo_sample.toml --raw
+            | lines
+            | skip 1
+            | first 1
+            | split-column "="
+            | get Column1
+            | trim
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "name");
+}