[syntax-errors] Detect duplicate keys in match mapping patterns (#17129)

Summary
--

Detects duplicate literals in `match` mapping keys.

This PR also adds a `source` method to `SemanticSyntaxContext` to
display the duplicated key in the error message by slicing out its
range.

Test Plan
--

New inline tests.

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

@@ -0,0 +1,19 @@
+match x:
+    case {"x": 1, "x": 2}: ...
+    case {b"x": 1, b"x": 2}: ...
+    case {0: 1, 0: 2}: ...
+    case {1.0: 1, 1.0: 2}: ...
+    case {1.0 + 2j: 1, 1.0 + 2j: 2}: ...
+    case {True: 1, True: 2}: ...
+    case {None: 1, None: 2}: ...
+    case {
+    """x
+    y
+    z
+    """: 1,
+    """x
+    y
+    z
+    """: 2}: ...
+    case {"x": 1, "x": 2, "x": 3}: ...
+    case {0: 1, "x": 1, 0: 2, "x": 2}: ...

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

@@ -0,0 +1,2 @@
+match x:
+    case {x.a: 1, x.a: 2}: ...