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-protocol/src/signature.rs

@@ -610,6 +610,37 @@ impl Signature {
             block_id,
         })
     }
+
+    pub fn formatted_flags(self) -> String {
+        if self.named.len() < 11 {
+            let mut s = "Available flags:".to_string();
+            for flag in self.named {
+                if let Some(short) = flag.short {
+                    let _ = write!(s, " --{}(-{}),", flag.long, short);
+                } else {
+                    let _ = write!(s, " --{},", flag.long);
+                }
+            }
+            s.remove(s.len() - 1);
+            let _ = write!(s, ". Use `--help` for more information.");
+            s
+        } else {
+            let mut s = "Some available flags:".to_string();
+            for flag in self.named {
+                if let Some(short) = flag.short {
+                    let _ = write!(s, " --{}(-{}),", flag.long, short);
+                } else {
+                    let _ = write!(s, " --{},", flag.long);
+                }
+            }
+            s.remove(s.len() - 1);
+            let _ = write!(
+                s,
+                "... Use `--help` for a full list of flags and more information."
+            );
+            s
+        }
+    }
 }
 
 #[derive(Clone)]