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-protocol/src/config.rs

@@ -33,6 +33,7 @@ pub struct Hooks {
     pub pre_execution: Option<Value>,
     pub env_change: Option<Value>,
     pub display_output: Option<Value>,
+    pub command_not_found: Option<Value>,
 }
 
 impl Hooks {
@@ -42,6 +43,7 @@ impl Hooks {
             pre_execution: None,
             env_change: None,
             display_output: None,
+            command_not_found: None,
         }
     }
 }
@@ -1529,9 +1531,10 @@ fn create_hooks(value: &Value) -> Result<Hooks, ShellError> {
                     "pre_execution" => hooks.pre_execution = Some(vals[idx].clone()),
                     "env_change" => hooks.env_change = Some(vals[idx].clone()),
                     "display_output" => hooks.display_output = Some(vals[idx].clone()),
+                    "command_not_found" => hooks.command_not_found = Some(vals[idx].clone()),
                     x => {
                         return Err(ShellError::UnsupportedConfigValue(
-                            "'pre_prompt', 'pre_execution', 'env_change', 'display_output'"
+                            "'pre_prompt', 'pre_execution', 'env_change', 'display_output', 'command_not_found'"
                                 .to_string(),
                             x.to_string(),
                             *span,