Add extern def which allows raw arguments (#8956)

# Description

Extends the `extern` syntax to allow commands that accept raw arguments.
This is mainly added to allow wrapper type scripts for external
commands.

This is an example on how this can be used:

```nushell
extern foo [...rest] { 
  print ($rest | str join ',' ) 
}
foo --bar baz -- -q -u -x
# => --bar,baz,--,-q,-u,-x
```

(It's only possible to accept a single ...varargs argument in the
signature)

# User-Facing Changes

No breaking changes, just extra possibilities.

# Tests + Formatting

Added a test for this new behaviour and ran the toolkit pr checker

# After Submitting

This is advanced functionality but it should be documented, I will open
a new PR on the book for that

Co-authored-by: Jelle Besseling <jelle@bigbridge.nl>

crates/nu-command/tests/commands/def.rs

@@ -141,3 +141,11 @@ fn def_with_paren_params() {
 
     assert_eq!(actual.out, "3");
 }
+
+#[test]
+fn extern_with_block() {
+    let actual =
+        nu!("extern foo [...rest] { print ($rest | str join ',' ) }; foo --bar baz -- -q -u -x");
+
+    assert_eq!(actual.out, "--bar,baz,--,-q,-u,-x");
+}