nu-cli refactor moving commands into their own crate nu-command (#2910)

* move commands, futures.rs, script.rs, utils

* move over maybe_print_errors

* add nu_command crate references to nu_cli

* in commands.rs open up to pub mod from pub(crate)

* nu-cli, nu-command, and nu tests are now passing

* cargo fmt

* clean up nu-cli/src/prelude.rs

* code cleanup

* for some reason lex.rs was not formatted, may be causing my error

* remove mod completion from lib.rs which was not being used along with quickcheck macros

* add in allow unused imports

* comment out one failing external test; comment out one failing internal test

* revert commenting out failing tests; something else might be going on; someone with a windows machine should check and see what is going on with these failing windows tests

* Update Cargo.toml

Extend the optional features to nu-command

Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>

crates/nu-command/tests/format_conversions/xml.rs

@@ -0,0 +1,16 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn table_to_xml_text_and_from_xml_text_back_into_table() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open jonathan.xml
+            | to xml
+            | from xml
+            | get rss.children.channel.children.0.item.children.0.guid.attributes.isPermaLink
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}