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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.
* Add Format for Stmt
* Implement basic module formatting
This implements formatting each statement in a module with a hard line break in between, so that we can start formatting statements.
Basic testing is done by the snapshots
