Add support for PatternMatchMapping formatting (#6836)

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## Summary

Adds support for `PatternMatchMapping` -- i.e., cases like:

```python
match foo:
    case {"a": 1, "b": 2, **rest}:
        pass
```

Unfortunately, this node has _three_ kinds of dangling comments:

```python
{  # "open parenthesis comment"
   key: pattern,
   **  # end-of-line "double star comment"
   # own-line "double star comment"
   rest  # end-of-line "after rest comment"
   # own-line "after rest comment"
}
```

Some of the complexity comes from the fact that in `**rest`, `rest` is
an _identifier_, not a node, so we have to handle comments _after_ it as
dangling on the enclosing node, rather than trailing on `**rest`. (We
could change the AST to use `PatternMatchAs` there, which would be more
permissive than the grammar but not totally crazy -- `PatternMatchAs` is
used elsewhere to mean "a single identifier".)

Closes https://github.com/astral-sh/ruff/issues/6644.

## Test Plan

`cargo test`

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/match.py

@@ -344,3 +344,58 @@ match foo:
         pass
     case [(*rest), (a as b)]:
         pass
+
+
+match foo:
+    case {"a": 1, "b": 2}:
+        pass
+
+    case {
+        # own line
+        "a": 1,  # end-of-line
+        # own line
+        "b": 2,
+    }:
+        pass
+
+    case {  # open
+        1  # key
+        :  # colon
+            value  # value
+    }:
+        pass
+
+    case {**d}:
+        pass
+
+    case {
+        **  # middle with single item
+        b
+    }:
+        pass
+
+    case {
+        # before
+        **  # between
+        b,
+    }:
+        pass
+
+    case {
+        1: x,
+        # foo
+        ** # bop
+        # before
+        b, # boo
+        # baz
+    }:
+        pass
+
+    case {
+        1: x
+        # foo
+        ,
+        **
+        b,
+    }:
+        pass