feat: add a command_not_found hook (#8314)

# Description
Add a `command_not_found` function to `$env.config.hooks`. If this
function outputs a string, then it's included in the `help`.

An example hook on *Arch Linux*, to find packages that contain the
binary, looks like:

```nushell
let-env config = {
  # ...
  hooks: {
    command_not_found: {
      |cmd_name| (
        try {
          let pkgs = (pkgfile --binaries --verbose $cmd_name)
          (
            $"(ansi $env.config.color_config.shape_external)($cmd_name)(ansi reset) " +
            $"may be found in the following packages:\n($pkgs)"
          )
        } catch {
          null
        }
      )
    }
    # ...
```

# User-Facing Changes
- Add a `command_not_found` function to `$env.config.hooks`.

# 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/system/run_external.rs

@@ -1,3 +1,4 @@
+use crate::hook::eval_hook;
 use fancy_regex::Regex;
 use itertools::Itertools;
 use nu_engine::env_to_strings;
@@ -324,9 +325,34 @@ impl ExternalCommand {
                             }
                         };
 
+                        let mut err_str = err.to_string();
+                        if engine_state.is_interactive {
+                            let mut engine_state = engine_state.clone();
+                            if let Some(hook) = engine_state.config.hooks.command_not_found.clone()
+                            {
+                                if let Ok(PipelineData::Value(Value::String { val, .. }, ..)) =
+                                    eval_hook(
+                                        &mut engine_state,
+                                        stack,
+                                        None,
+                                        vec![(
+                                            "cmd_name".into(),
+                                            Value::string(
+                                                self.name.item.to_string(),
+                                                self.name.span,
+                                            ),
+                                        )],
+                                        &hook,
+                                    )
+                                {
+                                    err_str = format!("{}\n{}", err_str, val);
+                                }
+                            }
+                        }
+
                         Err(ShellError::ExternalCommand {
                             label,
-                            help: err.to_string(),
+                            help: err_str,
                             span: self.name.span,
                         })
                     }