parser/add rest args to def (#2961)

* Add rest arg to def

This commit applied adds the ability to define the rest parameter of a def
command. It does not implement the functionality to expand the rest argument in
a user defined def function.

The rest argument has to be exactly worded "...rest".

Example after this PR is applied:

file test.nu
```shell
def my_command [
    ...rest:int # My rest arg
] {
    echo 1 2 3
}
```

```shell
> source test.nu
> my_command -h
Usage:
  > my_command ...args {flags}

Parameters:
  ...args: My rest arg

Flags:
  -h, --help: Display this help message
```

* Fix space in help on wrong side

crates/nu-source/src/meta.rs

@@ -48,14 +48,18 @@ impl Spanned<String> {
     ) -> impl Iterator<Item = &'a str> {
         items.map(|item| &item.item[..])
     }
-}
 
-impl Spanned<String> {
     /// Borrows the contained String
     pub fn borrow_spanned(&self) -> Spanned<&str> {
         let span = self.span;
         self.item[..].spanned(span)
     }
+
+    pub fn slice_spanned(&self, span: impl Into<Span>) -> Spanned<&str> {
+        let span = span.into();
+        let item = &self.item[span.start()..span.end()];
+        item.spanned(span)
+    }
 }
 
 pub trait SpannedItem: Sized {