Introduce AST nodes for PatternMatchClass arguments (#6881)

## Summary

This PR introduces two new AST nodes to improve the representation of
`PatternMatchClass`. As a reminder, `PatternMatchClass` looks like this:

```python
case Point2D(0, 0, x=1, y=2):
  ...
```

Historically, this was represented as a vector of patterns (for the `0,
0` portion) and parallel vectors of keyword names (for `x` and `y`) and
values (for `1` and `2`). This introduces a bunch of challenges for the
formatter, but importantly, it's also really different from how we
represent similar nodes, like arguments (`func(0, 0, x=1, y=2)`) or
parameters (`def func(x, y)`).

So, firstly, we now use a single node (`PatternArguments`) for the
entire parenthesized region, making it much more consistent with our
other nodes. So, above, `PatternArguments` would be `(0, 0, x=1, y=2)`.

Secondly, we now have a `PatternKeyword` node for `x=1` and `y=2`. This
is much more similar to the how `Keyword` is represented within
`Arguments` for call expressions.

Closes https://github.com/astral-sh/ruff/issues/6866.

Closes https://github.com/astral-sh/ruff/issues/6880.

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

@@ -458,3 +458,12 @@ match pattern_match_class:
         # e
     ):
         pass
+
+    case A(
+        # a
+        b # b
+        = # c
+        2 # d
+        # e
+    ):
+        pass