Add extra_usage messages for subcommand-only commands (#7594)

# Description

The message reads "You must use one of the following subcommands. Using
this command as-is will only produce this help message." and is added to
commands like `into`, `bytes`, `str`, etc.

# User-Facing Changes

See above.

# 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/src/formats/to/command.rs

@@ -21,6 +21,10 @@ impl Command for To {
             .input_output_types(vec![(Type::Nothing, Type::String)])
     }
 
+    fn extra_usage(&self) -> &str {
+        "You must use one of the following subcommands. Using this command as-is will only produce this help message."
+    }
+
     fn run(
         &self,
         engine_state: &EngineState,