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/lib.rs

@@ -14,6 +14,7 @@ mod filters;
 mod formats;
 mod generators;
 mod hash;
+pub mod hook;
 mod input_handler;
 mod math;
 mod misc;
@@ -26,6 +27,7 @@ mod shells;
 mod sort_utils;
 mod strings;
 mod system;
+pub mod util;
 mod viewers;
 
 pub use bits::*;
@@ -45,6 +47,7 @@ pub use filters::*;
 pub use formats::*;
 pub use generators::*;
 pub use hash::*;
+pub use hook::*;
 pub use math::*;
 pub use misc::*;
 pub use network::*;
@@ -55,6 +58,7 @@ pub use shells::*;
 pub use sort_utils::*;
 pub use strings::*;
 pub use system::*;
+pub use util::*;
 pub use viewers::*;
 
 #[cfg(feature = "dataframe")]