Detect assignment expressions before Python 3.8

This PR is the first in a series derived from #16308, each of which add support
for detecting one version-related syntax error from #6591. This one should be
the largest because it also includes a couple of additional changes:
1. the `syntax_errors!` macro, which makes adding more variants a bit easier
2. the `Parser::add_unsupported_syntax_error` method

Otherwise I think the general structure will be the same for each syntax error:
* Detecting the error in the parser
* Inline parser tests for the new error
* New ruff CLI tests for the new error

Because of the second point here, this PR is currently stacked on #16357.

As noted above, there are new inline parser tests, as well as new ruff CLI
tests. Once #16379 is resolved, there should also be new mdtests for red-knot,
but this PR does not currently include those.

Author: ntBre

crates/ruff_python_parser/resources/inline/err/walrus_before_py38.py

@@ -0,0 +1,2 @@
+# parse_options: { "target_version": "3.7" }
+if x := 1: ...

crates/ruff_python_parser/resources/inline/ok/walrus_after_py38.py

@@ -0,0 +1,2 @@
+# parse_options: { "target_version": "3.8" }
+if x := 1: ...

crates/ruff_python_parser/tests/snapshots/invalid_syntax@walrus_before_py38.py.snap

@@ -0,0 +1,60 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/err/walrus_before_py38.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..60,
+        body: [
+            If(
+                StmtIf {
+                    range: 45..59,
+                    test: Named(
+                        ExprNamed {
+                            range: 48..54,
+                            target: Name(
+                                ExprName {
+                                    range: 48..49,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 53..54,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                    body: [
+                        Expr(
+                            StmtExpr {
+                                range: 56..59,
+                                value: EllipsisLiteral(
+                                    ExprEllipsisLiteral {
+                                        range: 56..59,
+                                    },
+                                ),
+                            },
+                        ),
+                    ],
+                    elif_else_clauses: [],
+                },
+            ),
+        ],
+    },
+)
+```
+## Unsupported Syntax Errors
+
+  |
+1 | # parse_options: { "target_version": "3.7" }
+2 | if x := 1: ...
+  |    ^^^^^^ Syntax Error: Cannot use named assignment expression (`:=`) on Python 3.7 (syntax was added in Python 3.8)
+  |

crates/ruff_python_parser/tests/snapshots/valid_syntax@walrus_after_py38.py.snap

@@ -0,0 +1,53 @@
+---
+source: crates/ruff_python_parser/tests/fixtures.rs
+input_file: crates/ruff_python_parser/resources/inline/ok/walrus_after_py38.py
+---
+## AST
+
+```
+Module(
+    ModModule {
+        range: 0..60,
+        body: [
+            If(
+                StmtIf {
+                    range: 45..59,
+                    test: Named(
+                        ExprNamed {
+                            range: 48..54,
+                            target: Name(
+                                ExprName {
+                                    range: 48..49,
+                                    id: Name("x"),
+                                    ctx: Store,
+                                },
+                            ),
+                            value: NumberLiteral(
+                                ExprNumberLiteral {
+                                    range: 53..54,
+                                    value: Int(
+                                        1,
+                                    ),
+                                },
+                            ),
+                        },
+                    ),
+                    body: [
+                        Expr(
+                            StmtExpr {
+                                range: 56..59,
+                                value: EllipsisLiteral(
+                                    ExprEllipsisLiteral {
+                                        range: 56..59,
+                                    },
+                                ),
+                            },
+                        ),
+                    ],
+                    elif_else_clauses: [],
+                },
+            ),
+        ],
+    },
+)
+```