gh-138857: Improve error message for case outside of match (#138858)

* gh-138857: Improve error message for `case` outside of `match`

---------

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>

Lib/test/test_syntax.py

@@ -376,6 +376,42 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
+# Check incorrect "case" placement with specialized error messages
+
+>>> case "pattern": ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case 1 | 2: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case klass(attr=1) | {}: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case [] if x > 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case match: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case case: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> if some:
+...     case 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
+>>> case some:
+...     case 1: ...
+Traceback (most recent call last):
+SyntaxError: case statement must be inside match statement
+
 # But prefixes of soft keywords should
 # still raise specialized errors