[syntax-errors] Named expressions in decorators before Python 3.9 (#16386)

Summary
--

This PR detects the relaxed grammar for decorators proposed in [PEP
614](https://peps.python.org/pep-0614/) on Python 3.8 and lower.

The 3.8 grammar for decorators is
[here](https://docs.python.org/3.8/reference/compound_stmts.html#grammar-token-decorators):

```
decorators                ::=  decorator+
decorator                 ::=  "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
dotted_name               ::=  identifier ("." identifier)*
```

in contrast to the current grammar
[here](https://docs.python.org/3/reference/compound_stmts.html#grammar-token-python-grammar-decorators)

```
decorators                ::= decorator+
decorator                 ::= "@" assignment_expression NEWLINE
assignment_expression ::= [identifier ":="] expression
```

Test Plan
--

New inline parser tests.

crates/ruff_python_parser/src/parser/helpers.rs

@@ -43,3 +43,15 @@ pub(super) const fn token_kind_to_cmp_op(tokens: [TokenKind; 2]) -> Option<CmpOp
         _ => return None,
     })
 }
+
+/// Helper for `parse_decorators` to determine if `expr` is a [`dotted_name`] from the decorator
+/// grammar before Python 3.9.
+///
+/// [`dotted_name`]: https://docs.python.org/3.8/reference/compound_stmts.html#grammar-token-dotted-name
+pub(super) fn is_name_or_attribute_expression(expr: &Expr) -> bool {
+    match expr {
+        Expr::Attribute(attr) => is_name_or_attribute_expression(&attr.value),
+        Expr::Name(_) => true,
+        _ => false,
+    }
+}