[ty] Shrink size of AstNodeRef (#20028)

## Summary

Removes the `module_ptr` field from `AstNodeRef` in release mode, and
change `NodeIndex` to a `NonZeroU32` to reduce the size of
`Option<AstNodeRef<_>>` fields.

I believe CI runs in debug mode, so this won't show up in the memory
report, but this reduces memory by ~2% in release mode.

crates/ruff_python_parser/src/parser/mod.rs

@@ -133,7 +133,7 @@ impl<'src> Parser<'src> {
         ModExpression {
             body: Box::new(parsed_expr.expr),
             range: self.node_range(start),
-            node_index: AtomicNodeIndex::dummy(),
+            node_index: AtomicNodeIndex::NONE,
         }
     }
 
@@ -151,7 +151,7 @@ impl<'src> Parser<'src> {
         ModModule {
             body,
             range: TextRange::new(self.start_offset, self.current_token_range().end()),
-            node_index: AtomicNodeIndex::dummy(),
+            node_index: AtomicNodeIndex::NONE,
         }
     }