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/commands/path/mod.rs

@@ -0,0 +1,33 @@
+mod basename;
+mod dirname;
+mod exists;
+mod expand;
+mod extension;
+mod filestem;
+mod type_;
+
+use std::path::MAIN_SEPARATOR;
+
+/// Helper function that joins string literals with '/' or '\', based on host OS
+fn join_path_sep(pieces: &[&str]) -> String {
+    let sep_string = String::from(MAIN_SEPARATOR);
+    pieces.join(&sep_string)
+}
+
+#[cfg(windows)]
+#[test]
+fn joins_path_on_windows() {
+    let pieces = ["sausage", "bacon", "spam"];
+    let actual = join_path_sep(&pieces);
+
+    assert_eq!(&actual, "sausage\\bacon\\spam");
+}
+
+#[cfg(not(windows))]
+#[test]
+fn joins_path_on_other_than_windows() {
+    let pieces = ["sausage", "bacon", "spam"];
+    let actual = join_path_sep(&pieces);
+
+    assert_eq!(&actual, "sausage/bacon/spam");
+}