Correctly handle newlines after/before comments (#4895)

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## Summary

This issue fixes the removal of empty lines between a leading comment and the previous statement:

```python
a  = 20

# leading comment
b = 10
```

Ruff removed the empty line between `a` and `b` because:
* The leading comments formatting does not preserve leading newlines (to avoid adding new lines at the top of a body)
* The `JoinNodesBuilder` counted the lines before `b`, which is 1 -> Doesn't insert a new line

This is fixed by changing the `JoinNodesBuilder` to count the lines instead *after* the last node. This correctly gives 1, and the `# leading comment` will insert the empty lines between any other leading comment or the node.



## Test Plan

I added a new test for empty lines.

crates/ruff_python_formatter/src/lib.rs

@@ -355,11 +355,12 @@ Formatted twice:
     #[ignore]
     #[test]
     fn quick_test() {
-        let src = r#"
-while True:
-    if something.changed:
-        do.stuff()  # trailing comment
-other
+        let src = r#"AAAAAAAAAAAAA = AAAAAAAAAAAAA  # type: ignore
+
+call_to_some_function_asdf(
+    foo,
+    [AAAAAAAAAAAAAAAAAAAAAAA, AAAAAAAAAAAAAAAAAAAAAAA, AAAAAAAAAAAAAAAAAAAAAAA, BBBBBBBBBBBB],  # type: ignore
+)
 "#;
         // Tokenize once
         let mut tokens = Vec::new();