Add --directory (-D) flag to ls, list the directory itself instead of its contents (#5970)

* Avoid extending the directory without globs in `nu_engine::glob_from`

* avoid joining a `*` to the directory without globs

* remove checks on directory permission and whether it is empty

The previous implemention of `nu_engine::glob_from` will extend the
given directory even if it containes no glob pattern. This commit
overcomes lack of consistency with the function `nu_glob::glob`.

* Add flag -D to ls, to list the directory itself instead of its contents

* add --directory (-d) flag to ls

* correct the difference between the given path and the cwd

* set default path to `.` instead of `./*` when --directory (-d) flag is true

* add comments

* add an example

* add tests

* fmt

crates/nu-cli/tests/flag_completions.rs

@@ -14,15 +14,17 @@ fn flag_completions() {
     // Test completions for the 'ls' flags
     let suggestions = completer.complete("ls -", 4);
 
-    assert_eq!(12, suggestions.len());
+    assert_eq!(14, suggestions.len());
 
     let expected: Vec<String> = vec![
         "--all".into(),
+        "--directory".into(),
         "--du".into(),
         "--full-paths".into(),
         "--help".into(),
         "--long".into(),
         "--short-names".into(),
+        "-D".into(),
         "-a".into(),
         "-d".into(),
         "-f".into(),