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/resources/test/fixtures/ruff/trivia.py

@@ -0,0 +1,32 @@
+
+# Removes the line above
+
+a = 10  # Keeps the line above
+
+# Separated by one line from `a` and `b`
+
+b = 20
+# Adds two lines after `b`
+class Test:
+    def a(self):
+        pass
+        # trailing comment
+
+# two lines before, one line after
+
+c = 30
+
+while a == 10:
+    ...
+
+    # trailing comment with one line before
+
+# one line before this leading comment
+
+d = 40
+
+while b == 20:
+    ...
+    # no empty line before
+
+e = 50  # one empty line before