let UnknownFlag error list out available flags (#7443)

# Description
Fixes #6773.

```
/home/gabriel/CodingProjects/nushell〉ls -r                                                                                                                             12/12/2022 02:57:35 PM
Error: nu::parser::unknown_flag (link)

  × The `ls` command doesn't have flag `-r`.
   ╭─[entry #1:1:1]
 1 │ ls -r
   ·     ┬
   ·     ╰── unknown flag
   ╰────
  help: Available flags: --help(-h), --all(-a), --long(-l), --short-names(-s), --full-paths(-f), --du(-d), --directory(-D). Use `--help` for more information.
```

# User-Facing Changes

Different error for unknown flag.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -559,3 +559,17 @@ fn list_ignores_ansi() {
         assert!(actual.err.is_empty());
     })
 }
+
+#[test]
+fn list_unknown_flag() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+                ls -r
+            "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("Available flags: --help(-h), --all(-a),"));
+}