[ty] Implement the legacy PEP-484 convention for indicating positional-only parameters (#20248)

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/ty_python_semantic/src/ast_node_ref.rs

@@ -49,6 +49,12 @@ pub struct AstNodeRef<T> {
     _node: PhantomData<T>,
 }
 
+impl<T> AstNodeRef<T> {
+    pub(crate) fn index(&self) -> NodeIndex {
+        self.index
+    }
+}
+
 impl<T> AstNodeRef<T>
 where
     T: HasNodeIndex + Ranged + PartialEq + Debug,