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/other/keyword.rs

@@ -13,10 +13,10 @@ impl FormatNodeRule<Keyword> for FormatKeyword {
             arg,
             value,
         } = item;
+        // Comments after the `=` or `**` are reassigned as leading comments on the value.
         if let Some(arg) = arg {
             write!(f, [arg.format(), text("="), value.format()])
         } else {
-            // Comments after the stars are reassigned as trailing value comments
             write!(f, [text("**"), value.format()])
         }
     }