Handle keyword comments between = and value (#6883)

## Summary

This PR adds comment handling for comments between the `=` and the
`value` for keywords, as in the following cases:

```python
func(
    x  # dangling
    =  # dangling
    # dangling
    1,
    **  # dangling
    y
)
```

(Comments after the `**` were already handled in some cases, but I've
unified the handling with the `=` handling.)

Note that, previously, comments between the `**` and its value were
rendered as trailing comments on the value (so they'd appear after `y`).
This struck me as odd since it effectively re-ordered the comment with
respect to its closest AST node (the value). I've made them leading
comments, though I don't know that that's a significant improvement. I
could also imagine us leaving them where they are.

crates/ruff_python_formatter/src/expression/expr_dict.rs

@@ -40,6 +40,8 @@ impl Format<PyFormatContext<'_>> for KeyValuePair<'_> {
                 ])]
             )
         } else {
+            // TODO(charlie): Make these dangling comments on the `ExprDict`, and identify them
+            // dynamically, so as to avoid the parent rendering its child's comments.
             let comments = f.context().comments().clone();
             let leading_value_comments = comments.leading(self.value);
             write!(