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-cmd-lang/src/core_commands/extern_.rs

@@ -19,6 +19,7 @@ impl Command for Extern {
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
+            .optional("body", SyntaxShape::Block, "wrapper function block")
             .category(Category::Core)
     }