Make opening a directory list its contents (#3118)

* Make opening a directory enter it.

Not sure if this change is wanted, but I'm not sure what else opening a directory could mean.
And I find myself accidentally using `open <dir>` to mean `enter <dir>`

* Add example to open directory

* Open dir should list it's contents

* Update example description and fix style

crates/nu-command/tests/commands/open.rs

@@ -1,3 +1,4 @@
+use nu_test_support::fs::Stub::EmptyFile;
 use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, pipeline};
@@ -230,3 +231,24 @@ fn errors_if_file_not_found() {
         expected
     );
 }
+
+#[test]
+fn open_dir_is_ls() {
+    Playground::setup("open_dir", |dirs, sandbox| {
+        sandbox.with_files(vec![
+            EmptyFile("yehuda.txt"),
+            EmptyFile("jonathan.txt"),
+            EmptyFile("andres.txt"),
+        ]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open .
+                | count
+            "#
+        ));
+
+        assert_eq!(actual.out, "3");
+    })
+}