[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/node_key.rs

@@ -1,5 +1,7 @@
 use ruff_python_ast::{HasNodeIndex, NodeIndex};
 
+use crate::ast_node_ref::AstNodeRef;
+
 /// Compact key for a node for use in a hash map.
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, get_size2::GetSize)]
 pub(super) struct NodeKey(NodeIndex);
@@ -11,4 +13,8 @@ impl NodeKey {
     {
         NodeKey(node.node_index().load())
     }
+
+    pub(super) fn from_node_ref<T>(node_ref: &AstNodeRef<T>) -> Self {
+        NodeKey(node_ref.index())
+    }
 }