[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/resources/inline/err/decorator_expression_py38.py

@@ -0,0 +1,3 @@
+# parse_options: { "target-version": "3.8" }
+@buttons[0].clicked.connect
+def spam(): ...

crates/ruff_python_parser/resources/inline/err/decorator_named_expression_py37.py

@@ -0,0 +1,3 @@
+# parse_options: { "target-version": "3.7" }
+@(x := lambda x: x)(foo)
+def bar(): ...

crates/ruff_python_parser/resources/inline/ok/decorator_expression_dotted_ident_py38.py

@@ -0,0 +1,3 @@
+# parse_options: { "target-version": "3.8" }
+@buttons.clicked.connect
+def spam(): ...

crates/ruff_python_parser/resources/inline/ok/decorator_expression_eval_hack_py38.py

@@ -0,0 +1,3 @@
+# parse_options: { "target-version": "3.8" }
+@eval("buttons[0].clicked.connect")
+def spam(): ...

crates/ruff_python_parser/resources/inline/ok/decorator_expression_identity_hack_py38.py

@@ -0,0 +1,4 @@
+# parse_options: { "target-version": "3.8" }
+def _(x): return x
+@_(buttons[0].clicked.connect)
+def spam(): ...

crates/ruff_python_parser/resources/inline/ok/decorator_expression_py39.py

@@ -0,0 +1,5 @@
+# parse_options: { "target-version": "3.9" }
+@buttons[0].clicked.connect
+def spam(): ...
+@(x := lambda x: x)(foo)
+def bar(): ...