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/exists.rs

@@ -0,0 +1,53 @@
+use nu_test_support::fs::Stub::EmptyFile;
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+#[test]
+fn checks_if_existing_file_exists() {
+    Playground::setup("path_exists_1", |dirs, sandbox| {
+        sandbox.with_files(vec![EmptyFile("spam.txt")]);
+
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo spam.txt | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}
+
+#[test]
+fn checks_if_missing_file_exists() {
+    Playground::setup("path_exists_2", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo spam.txt | path exists"
+        );
+
+        assert_eq!(actual.out, "false");
+    })
+}
+
+#[test]
+fn checks_if_dot_exists() {
+    Playground::setup("path_exists_3", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo '.' | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}
+
+#[test]
+fn checks_if_double_dot_exists() {
+    Playground::setup("path_exists_4", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.test(),
+            "echo '..' | path exists"
+        );
+
+        assert_eq!(actual.out, "true");
+    })
+}