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/trivia.rs

@@ -146,6 +146,33 @@ pub(crate) fn lines_after(offset: TextSize, code: &str) -> u32 {
     newlines
 }
 
+/// Returns the position after skipping any trailing trivia up to, but not including the newline character.
+pub(crate) fn skip_trailing_trivia(offset: TextSize, code: &str) -> TextSize {
+    let rest = &code[usize::from(offset)..];
+    let mut iter = rest.char_indices();
+
+    while let Some((relative_offset, c)) = iter.next() {
+        match c {
+            '\n' | '\r' => return offset + TextSize::try_from(relative_offset).unwrap(),
+            '#' => {
+                // Skip the comment
+                let newline_offset = iter
+                    .as_str()
+                    .find(['\n', '\r'])
+                    .unwrap_or(iter.as_str().len());
+
+                return offset
+                    + TextSize::try_from(relative_offset + '#'.len_utf8() + newline_offset)
+                        .unwrap();
+            }
+            c if is_python_whitespace(c) => continue,
+            _ => return offset + TextSize::try_from(relative_offset).unwrap(),
+        }
+    }
+
+    offset + rest.text_len()
+}
+
 #[cfg(test)]
 mod tests {
     use crate::trivia::{lines_after, lines_before};